chore: transformers

[Rich-Harris]

While working on #12744 — a prerequisite for #12742 — it became clear that overhauling the way we transform state bindings couldn't wait any longer. The current code really is baffling — convoluted thickets of if statements, and logic scattered all over the place.

This PR introduces a completely new approach, expanding on #12530 and #12697. It adds the concept of ad hoc transformation functions to client transform state, where a transform is an object with a read method and (optionally) assign, mutate and update methods.

In other words we can declare all the logic for (for example) props or legacy reactive imports in one place, rather than having the setup logic over here, the getter logic over there, and the setter logic over in some other place. Distinguishing reassignments and mutations also makes everything much cleaner. (Updates fall back to assignments if no update function is provided.)

Draft because it's not quite finished:

• some tidying up that needs to happen
• add a test for this case
• add a snapshot test for the improved code generation about assignment patterns, which mostly addresses my concerns in chore: very minor perf improvement when fallback values are used #11921 (comment)
• changeset, because all big refactors come with a breakage risk

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