[whatwg/dom] Add reference target to shadow root (PR #1353)

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Reference Target is a feature to enable using IDREF attributes such as `for` and `aria-labelledby` to refer to elements inside a component's shadow DOM, while maintaining encapsulation of the internal details of the shadow DOM. The main goal of this feature is to enable ARIA to work across shadow root boundaries.

In this change, add the `referenceTarget` property to `ShadowRoot` and add a definition of `reference target` that's exported for use in other specs.

See the [reference target explainer](https://github.com/WICG/webcomponents/blob/gh-pages/proposals/reference-target-explainer.md).

- [ ] At least two implementers are interested (and none opposed):
   * https://github.com/mozilla/standards-positions/issues/1035 (no position stated yet)
   * https://github.com/WebKit/standards-positions/issues/356 (no position stated yet)
- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://wpt.fyi/results/shadow-dom/reference-target/tentative?label=experimental&label=master&aligned (still tentative)<!-- If these tests are tentative, link a PR to make them non-tentative. -->
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: …
   * WebKit: …
   * Deno (only for aborting and events): …
   * Node.js (only for aborting and events): …
- [ ] [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. -->


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<a href="https://whatpr.org/dom/1353.html" title="Last updated on Feb 4, 2025, 9:03 PM UTC (e06f903)">Preview</a> | <a href="https://whatpr.org/dom/1353/e6bb175...e06f903.html" title="Last updated on Feb 4, 2025, 9:03 PM UTC (e06f903)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/1353

-- Commit Summary --

  * Add reference target to shadow root

-- File Changes --

    M dom.bs (27)

-- Patch Links --

https://github.com/whatwg/dom/pull/1353.patch
https://github.com/whatwg/dom/pull/1353.diff

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