Add options for YouTube iframe title, including downloading

[gfscott]

Close #205

This PR adds several options related to YouTube embed titles:

• The default iframe title attribute is now configurable as an option. The default value remains the same.
• You can now opt to automatically download the actual title of the video from YouTube. This can be an accessibility win, since the iframe will better reflect the real content of the video. Note that activating this feature will cause network calls to YouTube for each embedded video — on large sites this could result in slow builds, or rate-limiting by YouTube, or both. If there's any failure in fetching the title, it will fall back to the default value.

How to use it

This simplified example shows the new title options in action:

eleventyConfig.addPlugin(embedYouTube, {
  title: "Custom iframe title wooooo",    // All YouTube iframes get this title by default
  titleOptions: {
    download: true,                       // Download the titles from YouTube
    cacheDuration: '1h'                   // Cache titles for one hour
  }
});

Notes on implementation

• Javascript’s native .replace prototype is synchronous, which makes awaiting the title impossible, so we now use the string-replace-async package instead.
  • Currently it has to be pinned to v2.0.0 because that's the most recent version that supports CJS modules. We can update later when Eleventy 3 ships.
  • I do prefer using native JS whenever possible but this library has decent usage and all tests continue to pass.
• The nature of this update makes it kind of an all-or-nothing change; the plugin is asynchronous for everyone as of this release, whether they turn on titleOptions.download or not. I worry a smidge about backward compatibility as a result, but hey, that's what semantic versioning is for, I guess.
• One nice effect is that making the pattern-replacements async is that they can also be made concurrent, so that could be a performance win.
• I've added some new tests just to really convince myself that the new behavior is truly equivalent to .replace(). It seems to be at parity.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: ac4c936fe523b01927d4ce293be1cdb40e516f27

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

| Name | Type | | -------------------------------- | ----- | | eleventy-plugin-youtube-embed | Minor | | eleventy-plugin-embed-everything | Minor |

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (cbab6b4) 100.00% compared to head (ac4c936) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #224 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 7 7 Lines 158 172 +14 ========================================= + Hits 158 172 +14 ```

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