chore: form state rework

[dummdidumm]

We now use $state instead of $state.raw for the internal representation of value()/set(...). This has several advantages:

• we can get rid of the version code which became quite a bit
• you can put a $state object yourself into set(...) and then just push to it, making array operations very easy (closes Request: Add field array helpers (push, pop, splice, etc) #14662)
• fine-grained without any additional footwork

The reason I made this $state.raw originally is so that fields.x.value() would fire even if something within x would update, but honestly that's a bad reason looking back. You can get that behavior by running JSON.stringify on the result, or you just access the properties you care about and it will all take care of it automatically.

The one drawback is that prototype pollution prevention became a tiny but trickier, we now have to check for a set of keys. We should consider changing $state in Svelte itself to also proxify Object.create(null); maybe we can make that change when the experimental async flag is on?

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[dummdidumm]

Didn't quite understand why that was necessary here; no test fails without it

[Rich-Harris]

woah!