[whatwg/dom] Collapse range before mutations in deleteContents and extract (PR #1452)

Collapse range before mutations in deleteContents and extract

Move the step that sets the range's start and end to (newNode, newOffset) from after DOM mutations to before them, in both `deleteContents()` and the `extract` algorithm. The DOM's built-in live range maintenance mechanisms (`live range pre-remove steps` and `replace data` range adjustments) keep the range valid through subsequent operations. Collapsing after mutations is unreliable because script can run during removal (via removing steps) and modify the DOM, invalidating the pre-computed values.

Fixes #1446.

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- [ ] At least two implementers are interested (and none opposed):
   * …
   * …
- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://github.com/web-platform-tests/wpt/pull/58008
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: https://issues.chromium.org/issues/486922855
   * Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2018839
   * WebKit: pending (account approval required on bugs.webkit.org)
- [x] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: https://github.com/mdn/content/issues/43230
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use.

(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/1452

-- Commit Summary --

  * Collapse range before mutations in deleteContents and extract

-- File Changes --

    M dom.bs (30)

-- Patch Links --

https://github.com/whatwg/dom/pull/1452.patch
https://github.com/whatwg/dom/pull/1452.diff

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