feat: allow user to provide custom pseudo elements to be captured

[SUCHMOKUO]

Hi! Thanks for maintaining this great library!

I've tried it and just found that it might be great if we can control what pseudo elements will be included in the captured image rather than just ::before and ::after, sometimes we might want to include some non-standard but useful ones, like ::-webkit-scrollbar and ::-webkit-scrollbar-thumb, so I created this PR to add a new option includedPseudoElements to allow user to customize the rule of the included pseudo elements.

Hope this can be helpful :)

Description

1. Add new option includedPseudoElements.
2. Refactor clonePseudoElements to receive the includedPseudoElements list and clone all the pseudo elements in the list.
3. Add test for this option.
4. Update README.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Enhancement (changes that improvement of current feature or performance)
• [ ] Refactoring (changes that neither fixes a bug nor adds a feature)
• [ ] Test Case (changes that add missing tests or correct existing tests)
• [ ] Code style optimization (changes that do not affect the meaning of the code)
• [ ] Docs (changes that only update documentation)
• [ ] Chore (changes that don't modify src or test files)

Self Check before Merge

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[vivcat[bot]]

👋 @SUCHMOKUO

💖 Thanks for opening this pull request! 💖

Please follow the contributing guidelines. And we use semantic commit messages to streamline the release process.

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• fix: don't overwrite prevent_default if default wasn't prevented
• feat: add graph.scale() method
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[codecov[bot]]

Codecov Report

Base: 62.93% // Head: 62.93% // No change to project coverage :thumbsup:

Coverage data is based on head (f6363fc) compared to base (b751cbf). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #376 +/- ## ======================================= Coverage 62.93% 62.93% ======================================= Files 10 10 Lines 580 580 Branches 143 143 ======================================= Hits 365 365 Misses 151 151 Partials 64 64 ``` | [Impacted Files](https://codecov.io/gh/bubkoo/html-to-image/pull/376?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96) | Coverage Δ | | |---|---|---| | [src/util.ts](https://codecov.io/gh/bubkoo/html-to-image/pull/376?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96#diff-c3JjL3V0aWwudHM=) | `62.80% <ø> (ø)` | | | [src/clone-node.ts](https://codecov.io/gh/bubkoo/html-to-image/pull/376?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96#diff-c3JjL2Nsb25lLW5vZGUudHM=) | `71.42% <100.00%> (ø)` | | | [src/clone-pseudos.ts](https://codecov.io/gh/bubkoo/html-to-image/pull/376?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96#diff-c3JjL2Nsb25lLXBzZXVkb3MudHM=) | `29.16% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96)

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