[core] Support MutationObserver behind a flag about three-elements

[hmans]

Also from my side let me voice my commitment to re-enable this in some fashion in a future (ideally the 0.4) release. Things I want to explore:

• making it opt-in by setting an observe attribute (or similar) either on three-game or individual elements (or element branches.)
• finding out if we can use a single MutationObserver for a full tree (and not just individual elements)
• possibly implementing our own logic that we can configure to not run every frame, but only every n frames.

[hmans]

I've done some experimenting (see the extremely WIP Draft PR above).

The good news is that it's possible to just have one MutationObserver observing the entire tree of a <three-game> instance (instead of having one MutationObserver instance per element.)

The bad news is that this doesn't seem to have any noticeable impact on the kind of bad performance we've had with MutationObservers enabled.

Still, as a purely opt-in feature, this is okay, and I will continue to work on it.

[RangerMauve]

Is there a way to import this version of the library using skypack?

[RangerMauve]

Trying to place some models in a scene, and it's a little frustrating to have to edit the source and reload each time. 😅

[CodyJasonBennett]

If nothing else, the parcel setup on codesandbox works very well for this (https://js.new).