Auto merge of #14971 - asajeffrey:script-track-active-documents, r=cbrewster

Constellation informs script about documents becoming inactive, active or fully active.

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This PR replaces the current freeze/thaw mechanism by messages from the constellation to script informing it about when documents become inactive, active or fully active.

This means we can now implement |Document::is_active()| which is used in |document.write|.

This PR also changes how timers work: previously they were initialized running, and were then frozen/thawed. This means there was a transitory period when timers were running even though the document was not fully active. With this PR, timers are initially suspended, and are only resumed when the document is made fully active.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14876
- [X] These changes do not require tests because it's an interal refactoring.

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