[whatwg/dom] Add "precustomized" custom element state (#894)

Add the "precustomized" state needed by the [HTML PR](https://github.com/whatwg/html/pull/5909) that prevents usage of `attachInternals()` prior to the constructor being run.

- [X] At least two implementers are interested (and none opposed):
   * Chromium: me
   * Safari: based on [this comment](https://github.com/w3c/webcomponents/issues/871#issuecomment-690879744)
- [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/25402
- [X] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chrome: Tracked as part of the declarative Shadow DOM [tracking bug](https://crbug.com/1042130).
   * Firefox: none yet
   * Safari: none yet


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

  https://github.com/whatwg/dom/pull/894

-- Commit Summary --

  * Add "precustomized" state

-- File Changes --

    M dom.bs (18)

-- Patch Links --

https://github.com/whatwg/dom/pull/894.patch
https://github.com/whatwg/dom/pull/894.diff

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