[whatwg/dom] Add "mode locked" flag to Document, plus an algorithm to use it (#989)

This goes along with https://github.com/whatwg/html/pull/6745. This PR adds a "mode locked" flag to Document, which can be used to lock the [mode](https://dom.spec.whatwg.org/#concept-document-mode), so that it cannot be later changed.

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

  https://github.com/whatwg/dom/pull/989

-- Commit Summary --

  * Add "mode locked" flag and algorithm to use it

-- File Changes --

    M dom.bs (18)

-- Patch Links --

https://github.com/whatwg/dom/pull/989.patch
https://github.com/whatwg/dom/pull/989.diff

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