fix: granular updates of field.value()

[Rich-Harris]

This closes a TODO, and insodoing fixes an actual bug — if you edit a form that contains a select, the value getter will re-run (because input has changed) and nuke the select because the model value is typically undefined to start with.

Unfortunately it doesn't quite work, because an effect doesn't depend on the $derived values that were created inside it. So when you do {field.value()}, you're creating a derived and then immediately (trying to) depend on it. I'm not immediately sure what footwork would be required to work around that, or if it indicates that it should be possible for an effect to depend on a derived it created. So I'm leaving this in draft for now.

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[Rich-Harris]

Worked around it with a time-worn hack: maintaining a set of version sources created outside the effects in question

[Rich-Harris]

As @dummdidumm just noted elsewhere, the $derived thing does work when experimental.async is true, but only then (because we changed the behaviour around deriveds in async mode for exactly this reason). But the solution in 1cd0ab4 works in both cases

[SLAN14]

Hey @Rich-Harris, this PR is causing issues while building in svelte v4