chore: Bindable and Svelte 5 interop

[dummdidumm]

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[dummdidumm]

@jasonlyu123 this is still not fully finished, but let me know what you think of the approach:

• Svelte will provide a Bindable type to mark things as being able to bind: to
• We're going to wrap bindings with __sveltets_2_binding which gives back the Binding type. This means you'll get type errors when you're binding to something that cannot be bound to (Bindable missing)
• ExportedNames.ts is enhanced to mark which things are bindable (so their types are wrapped with Bindable so you don't have to do it yourself) when using $props()
• Due to backwards compat we gotta jump through some hoops: By adding & "_explicit_" to the props type we're marking it as "only things typed with Bindable can be used with bind:". That means all the existing d.ts files are implicitly all bindable. The alternative of not doing this would mean we would have to wait until Svelte 7 until we can give editor squigglies, because until then people could have old legacy typings Update: Thought of a better way: we're going to add an explicit constructor where we don't do the "wrap all with bindings" stuff which the SvelteComponent class type will do by default.
• Massage error messages to get rid of weird types and type helpers making the error messages more confusing than they need to be