Migrate `CustomRender` to `HtmlExtension`

[Sub6Resources]

Closes #1142

Getting started on this. Here's a task list for myself. Any additional feedback/suggestions are still welcome.

• [x] Write HtmlExtension class
• [x] Write ExtensionContext class
• [x] Integrate HtmlExtension into the Html widget flow
• [x] Write TagExtension helper class
• [x] Write MatcherExtension helper class
• [x] Rewrite flutter_html_audio to use HtmlExtension
• [x] Rewrite flutter_html_iframe to use HtmlExtension
• [x] Rewrite flutter_html_math to use HtmlExtension
• [x] Rewrite flutter_html_svg to use HtmlExtension
• [x] Rewrite flutter_html_table to use HtmlExtension
• [x] Rewrite flutter_html_video to use HtmlExtension
• [x] Write ImageExtension class
• [x] Bump version to 3.0.0-beta.1
• [x] Add migration guide
• [ ] Add better documentation for HtmlExtension

Fixes #1137 Closes #717 Closes #1134 Closes #1105

Fixes #1206. Fixes #1199. Fixes #1180. Fixes #1113

[codecov[bot]]

Codecov Report

Patch coverage: 77.09% and project coverage change: +5.76 :tada:

Comparison is base (c75e0df) 51.40% compared to head (3ed46cf) 57.16%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1176 +/- ## ========================================== + Coverage 51.40% 57.16% +5.76% ========================================== Files 19 36 +17 Lines 2640 2610 -30 ========================================== + Hits 1357 1492 +135 + Misses 1283 1118 -165 ``` | [Impacted Files](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker) | Coverage Δ | | |---|---|---| | [lib/src/anchor.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9hbmNob3IuZGFydA==) | `66.66% <ø> (ø)` | | | [lib/src/extension/helpers/image\_extension.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9leHRlbnNpb24vaGVscGVycy9pbWFnZV9leHRlbnNpb24uZGFydA==) | `0.00% <0.00%> (ø)` | | | [lib/src/extension/helpers/image\_tap\_extension.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9leHRlbnNpb24vaGVscGVycy9pbWFnZV90YXBfZXh0ZW5zaW9uLmRhcnQ=) | `0.00% <0.00%> (ø)` | | | [lib/src/style.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9zdHlsZS5kYXJ0) | `84.87% <0.00%> (ø)` | | | [lib/src/utils.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy91dGlscy5kYXJ0) | `77.27% <ø> (ø)` | | | [lib/src/builtins/details\_element\_builtin.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9idWlsdGlucy9kZXRhaWxzX2VsZW1lbnRfYnVpbHRpbi5kYXJ0) | `11.11% <11.11%> (ø)` | | | [lib/src/tree/styled\_element.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy90cmVlL3N0eWxlZF9lbGVtZW50LmRhcnQ=) | `23.68% <23.68%> (ø)` | | | [lib/src/extension/helpers/matcher\_extension.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9leHRlbnNpb24vaGVscGVycy9tYXRjaGVyX2V4dGVuc2lvbi5kYXJ0) | `53.84% <53.84%> (ø)` | | | [lib/src/builtins/interactive\_element\_builtin.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9idWlsdGlucy9pbnRlcmFjdGl2ZV9lbGVtZW50X2J1aWx0aW4uZGFydA==) | `54.76% <54.76%> (ø)` | | | [lib/src/extension/extension\_context.dart](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker#diff-bGliL3NyYy9leHRlbnNpb24vZXh0ZW5zaW9uX2NvbnRleHQuZGFydA==) | `55.17% <55.17%> (ø)` | | | ... and [20 more](https://app.codecov.io/gh/Sub6Resources/flutter_html/pull/1176?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Whitaker) | |

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[PritamSangani]

Hi @Sub6Resources - is there anything the community can do to help get this PR finished and a new major version released?

[Sub6Resources]

Continuing from #1153

[Sub6Resources]

Migrating to Extension has touched pretty much all of the codebase, so I've taken the opportunity to remove some technical debt and fix bugs as I found them.

Here's a summary of major changes in this PR so far (this list is a work in progress):

• Removed SelectableHtml (users should now use SelectionArea provided by Flutter). See #1153. Fixes #1137
• Upgrades dependencies in all the sub-packages (fixes #1206, fixes #1113)
• Improvements to <table> code. Fixes #1199
• Simplify <img> code. Removes default loading animation. (fixes #1180)

[Sub6Resources]

This pull request is ready. Just needs some additional documentation on the Wiki. Anybody that's just stopping in, please review! I'd love to get this release out sometime tomorrow!

[PritamSangani]

There was a comment made on the initial extension api RFC issue about the name of the api, with Extension potentially being confusing and leading to conflicts with it being quite generic. A suggestion was made to change to something like HtmlExtension. I was wondering if you had any thoughts on this.

re: https://github.com/Sub6Resources/flutter_html/issues/1142#issuecomment-1333679864

[Sub6Resources]

@PritamSangani Thanks for pointing that out. I had forgotten about that comment (although I had named all the extension packages as if I had already made the change -- IframeHtmlExtension for example).

I agree! Just updated from Extension to HtmlExtension

[Sembauke]

congrats 🥳