fix(language-core): preserve declared setup binding types in template exposure

[serkodev]

Fix #5958

---

Avoid typeof flow narrowing in __VLS_SetupExposed for mutable bindings with explicit type annotations and initializers (e.g. let x: boolean = true).

Generate declared-type-based exposure only for that subset with a helper class field initializer pattern, while keeping existing typeof behavior for other bindings to preserve control-flow typing.

POC

let test: boolean = true;

type t = typeof test
//   ^? true

class Exposed {
  ['test'] = test;
}

type t2 = Exposed['test']
//   ^? boolean

TS Playground

[KazariEX]

I think it would be better to use

type __VLS_SetupExposed = import("vue").ShallowUnwrapRef<{
  foo: ReturnType<() => typeof foo>;
}>;

to avoid creating an extra type.

[serkodev]

Oh, that’s a better way, thanks!

[KazariEX]

After #5982 it can be further simplified.