breaking: make non-bindable props read-only

[Rich-Harris]

Now that $state.link exists, we can make props a bit less weird. Rather than allowing them to be written to, causing a temporary divergence between the parent and child values, we can make that behaviour explicit with $state.link and make it a compile error to reassign a non-bindable prop.

This change likely also creates some opportunities for simplification — leaving in draft status until I've looked into that

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Latest commit: 73a3895

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[Rich-Harris]

Closing, as $state.link was removed in #12942