core: refine typing of isObject<T>

[paul-marechal]

The template parameter for isObject is currently used to force cast the value passed to the function, although there is no guarantee nor check done to ensure that this is really the case.

This commit updates the typings so that the type passed to isObject is used to guide TypeScript during subsequent field type checking, assuming each field value is unknown and must also be type checked.

Example:

interface MyType {
    foo: string
}

const unknownValue: unknown = { foo: 2 };

if (isObject<MyType>(unknownValue)) {
    // We enter this branch because `unknownValue` is indeed an object,
    // and TypeScript now knows we might want to access the `foo` field
    // from our interface. But we can't yet assume to know the type of
    // `foo`, so it is still typed as `unknown` and we must check:
    if (isString(unknownValue.foo)) {
        // We won't get in this branch. But if we did, then `foo` is
        // now properly typed (and checked) as `string`.
    }
}

Fix #12253
Fix #12250

Close #12251

How to test

Should compile.

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[eneufeld]

@paul-marechal ,
I think this makes #12251 obsolete, do you agree?

[tsmaeder]

I've checked that we're finding the bug @eneufeld detected with this change.

[paul-marechal]

I think this makes #12251 obsolete, do you agree?

Yes and no. While this PR might fix the issue you found, I still think it would be for the best to update the ProblemMarker.is function such as:

export namespace ProblemMarker {
    export function is(node: unknown): node is ProblemMarker {
        return Marker.is(node) && node.kind === PROBLEM_KIND;
    }
}

Just for good measure. edit: I pushed a commit that does that.

[msujew]

I'm happy with the changes 👍