[core] Similar to `tick` `late-tick` etc., also offer `mount` and `unmount` (or similar) about three-elements

[CodyJasonBennett]

Not sure exactly what mount implies but appear can be misleading if objects do not leave view.

[hmans]

I'm currently using the term "mount" to describe the point at which a ThreeElement is not only connected (in the web components sense), but has also initialized everything its Three.js-specific stuff, which typically happens within a microtask enqueued to be executed after all the elements are finished connecting, so also calling this event "mount" or "mounted" would allow us to continue using that terminology. (Same thing for "remove", which I specifically did not call "unmount" on purpose, because at the point that it is run, the element itself has already been disconnected.)