SlotFill: rewrite base Slot to functional, unify rerenderable refs

[jsnajdr]

This is the last SlotFill refactor that I originally included in #67000 and later decided to merge one-by-one.

The main outcome is refactoring the (base) Slot component from a class component to functional. The componentDidMount/DidUpdate code is converted to a useEffect. The render code is the same, it just moved from the render() method to the functional component body.

The class version registered its own instance, i.e., this, to the slot registry. But functional components don't have instances like this. That's why I'm creating a Renderable interface with a rerender method, storing a constant per-instance value to a ref, and registering this interface instead.

The values in the context that were previously of type Component< BaseSlotComponentProps > are now Rerenderable.

It turns out that exactly the same Renderable interface is already used by bubbles-virtually Fill, so I'm reusing the same code (useForceUpdate).

Last minor update I did is renaming the SlotFillBubblesVirtuallySlotRef to SlotRef, to make the name shorter and prevent the code that uses it from having too many linebreaks.

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Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[tyxla]

It would be nice to get a second opinion, just in case, but it's not a blocker :)

I'm actually taking a look right now.