Make starting animations idempotent and improve runner performance

[dweymouth]

Description:

This PR does the following:

• Introduces a new public API animation.State() which returns the state of an animation (not started, running, stopped). We use this internally as well
• Makes starting animations idempotent. Starting an animation again while it is running is a no-op.
• Improves performance of the animation runner:
  • Start is O(1) amortized
  • Stop is O(1) (down from O(n))
  • the runner is zero-alloc

Fixes #4445

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.
• Check for binary size increases when importing new modules.

[dweymouth]

Wrong base; will close and re-open a new PR