[whatwg/dom] Add Scoped Custom Element Registries (PR #1341)

**Do not comment directly on this PR while it is in draft state.** Use #1339 instead.

HTML PR: ...

Tests: ...

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- [ ] At least two implementers are interested (and none opposed):
   * WebKit
   * …
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(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/1341

-- Commit Summary --

  * Add Scoped Custom Element Registries

-- File Changes --

    M dom.bs (315)

-- Patch Links --

https://github.com/whatwg/dom/pull/1341.patch
https://github.com/whatwg/dom/pull/1341.diff

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