chore: remove exposable

[Rich-Harris]

This is a more conservative approach to simplifying props than #9767 — I'm going to work in multiple manageable (and hopefully uncontroversial) stages rather than a few chonky PRs.

Right now, we have some clever-but-complicated machinery around bindings: instead of having a setter on the props object, in some cases we 'expose' the signal via the getter so that it can be set directly by the child.

This might have made sense in a world where we would set the prop on every mutation and didn't want to have chains of binding setters between the mutation and the ultimate signal update, but in a #9739 world it's just wantonly confusing (to this grug-brained developer, at least). It's much simpler to just always provide a setter. Doing so allows us to delete a bunch of internal library code and get rid of some global state that we need to check on every single get.

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[Rich-Harris]

Oh, and I tweaked stuff around prop_source so that it takes a flag of options rather than taking immutable and call_default_value as separate args. This will pay dividends when we add a runes flag for disallowing fallbacks on bound props

[dummdidumm]

This change makes sense to me - it may have tiny performance implications but these are so rare that it doesn't warrant the additional logic, and this gets code size down a bit, removes some global flags (always good), and in the long run we don't need it anyway - win.

[dummdidumm]

Why can we remove this? I'm a bit surprised that no tests fail because removing this seems unrelated. Is it because the parent now always decides what to do with the update?