V1: mount/unmount concept and automatic scheduling

[dvoytenko]

Partial for #31915.
Partial for #31540.

Key changes:

• The mount/unmount callbacks are added to a V1 element API. They are called automatically on first build, attach/detach from DOM, manual unmount, etc. They are similar to layout/unlayout callbacks, but meant to be executed with build and more "like build, but reversible".
• Builder now calls mountInternal() on an element. Consequently it's renamed to Scheduler.
• BaseElement.load() is added to make build->load states a little less fragile.
• The layoutCallback, unlayoutCallback, pauseCallback, and resumeCallback in the CustomElement have been deprecated.

TODO:

• Complete the remaining tests

[jridgewell]

One last question, otherwise I think it's good.