[whatwg/dom] Add descendant changed steps (PR #1514)

HTML needs to know when the label of an option element changes, which depends on the data of its descendant Text nodes and the alt attributes of its descendant img elements. Mutation observers cannot be used for that from a specification.

This adds a hook that runs for each inclusive ancestor of a node whose children or data change, along with a warning on how implementations are expected to scope it. Specifications that also depend on an attribute can signal a descendant change for it themselves.

- [ ] At least two implementers are interested (and none opposed):
   * WebKit
   * …
- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * This is exercised through the HTML change: [html/semantics/forms/the-option-element/dynamic-content-change-rendering.html](https://wpt.fyi/results/html/semantics/forms/the-option-element/dynamic-content-change-rendering.html) and https://github.com/web-platform-tests/wpt/pull/64050
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: …
   * WebKit: https://bugs.webkit.org/show_bug.cgi?id=301858 (fixed)
   * Deno (only for aborting and events): N/A
   * Node.js (only for aborting and events): N/A
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: N/A, nothing web-exposed on its own
- [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 [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

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

  https://github.com/whatwg/dom/pull/1514

-- Commit Summary --

  * Add descendant changed steps

-- File Changes --

    M dom.bs (46)

-- Patch Links --

https://github.com/whatwg/dom/pull/1514.patch
https://github.com/whatwg/dom/pull/1514.diff

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