Improve type narrowing for `.includes`

Given the following code:

(["a", "b", "c"] as const).includes(SOME_VALUE as 'a' | 'b' | 'd' )

we get a Typeguard that SOME_VALUE is 'a' | 'b' | 'c' — but in this case we know more about: we know it is not 'c' so it would be great to not have 'c' show up in the result again. The typeguard should narrow to 'a' | 'b'

One naive way to implement it would be something like:

interface ReadonlyArray<T> {
  includes<TSearch extends T | (TSReset.WidenLiteral<T> & {})>(
    searchElement: TSearch,
    fromIndex?: number,
  ): searchElement is T & TSearch;
}

Diff:

interface ReadonlyArray<T> {
-  includes(
-    searchElement: T | (TSReset.WidenLiteral<T> & {}),
+  includes<TSearch extends T | (TSReset.WidenLiteral<T> & {})>(
+    searchElement: TSearch,
    fromIndex?: number,
-  ): searchElement is T;
+  ): searchElement is T & TSearch;
}

Is there any reason this shouldn't be done? If not I'd be glad to open a PR.

Start by reviewing the ReadonlyArray.includes signature shown in the issue and the project's existing TypeScript type tests or augmentation entry point. Verify how the proposed generic signature affects the example, especially the resulting type guard, and add coverage demonstrating that 'c' is excluded from the narrowed result.