Components: Handle multiple Slots by same name

[aduth]

Fixes #12355
Alternative to: #12580

This pull request seeks to resolve an issue where multiple Slot instances using the same name can conflict:

• Previously, both would render the Fills, except when bubblesVirtually is true, since it uses createPortal to render to a single node
  • Now, only the last-registered Slot will render the Fills
• Previously, if an earlier-registered Slot unmounted, it would cause the last-registered Slot to become destroyed
  • Now, a slot is only removed from context if it is the last-registered instance

Testing instructions:

Repeat Steps to Reproduce from #12355, verifying that no duplicate toolbar is shown.

Verify there are no regressions in the behavior of Slot / Fill, both for bubblesVirtually true (e.g. Popovers) and false (e.g. BlockToolbars).

Ensure unit tests pass:

npm run test-unit

[johngodley]

I've been unable to reproduce the original problem on master. I guess something changed that's affected the order in which block duplication happens, and the slot's aren't used in the same way.

If I try code from before this PR then the problem is reproducible, and this PR does fix it. I've tried to test other instances of Slot/Fill, and haven't seen any side-effects so far.

Regardless of whether the block duplication shortcut causes a problem in master I think the fix is still important. Nice 👍