Html: Add a pretty diagram for the references in "Garbage collection and browsing contexts"

[domenic]

https://html.spec.whatwg.org/#garbage-collection-and-browsing-contexts

Something like this:

[tabatkins]

I was bored, here you go: https://www.xanthir.com/pictures/gc-diagram.svg

[domenic]

@annevk we may want to tackle this while it's all fresh in our heads. Probably after all your ongoing cleanup.

[annevk]

Ideally after #4361.

[domenic]

This is my current attempt after all the recent changes:

Dotted lines are "weak references", e.g. document's BC is defined as "the BC whose session history has this document", and environment settings object's BC and responsible document are defined via algorithms which indirect through the ESO's Window object.

I'm not super-happy with that version in general; probably needs some pruning or focusing or similar.

[annevk]

4390 will further complicate this.

(The agent in your diagram will belong to an agent cluster that's hold by a map in the BCG, btw.)

[tabatkins]

Ping me when you're done enough to put something into the spec, and I'll adjust my previous diagram.