[whatwg/dom] Add serializable to ShadowRoot and "attach a shadow root" (PR #1256)

Add `getHTML()` and the `serializable` concept for shadow roots. See https://github.com/whatwg/html/issues/8867#issuecomment-1856696628 for the rough consensus on how this should work, plus the few comments below that with modifications.

- [X] At least two implementers are interested (and none opposed):
   * Chromium
   * [WebKit](https://github.com/whatwg/html/issues/8867#issuecomment-1910624523)
   * 
- [X] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://wpt.fyi/results/shadow-dom/declarative/gethtml.tentative.html
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: https://crbug.com/41490936
   * Gecko: …
   * WebKit: …
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: …
- [X] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use. <!-- If you created this PR from a single commit, Github copied its message. Otherwise, you need to add a commit message yourself. -->

See the corresponding HTML spec PR here: https://github.com/whatwg/html/pull/10139

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/1256

-- Commit Summary --

  * Add serializable to ShadowRoot and "attach a shadow root"

-- File Changes --

    M dom.bs (15)

-- Patch Links --

https://github.com/whatwg/dom/pull/1256.patch
https://github.com/whatwg/dom/pull/1256.diff

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