Fix #25172: Add narrowing for unknown with triple equals

[jack-williams]

Fixes #25172.

Narrows an identifier of type unknown when strict compared (===) with a value of primitive, non-primitive, or object type.

• Comparing with a primitive/non-primitive type narrows to the type of the operand, retaining any literal types.
• Comparing with an object type narrows to the default non-primitive type object.

Limitations.

• Does not work with == because we would need to consider coercion rules.
• Does not work with intersection or union:
  • Intersection: Branding tricks like number & "age" make it hard to tell the actual runtime type. We could inherit the intersection type but this might violate branding expectations.
  • Union: To narrow from a union would require traversing the union and collecting the set possible target types. The performance impacts of this aren't clear, and I'm not sure how often this happens anyway.

[jack-williams]

Shall I revive this and remerge master?

[jack-williams]

cc @RyanCavanaugh

Not many people seem to have run into this recently, but if you're still interested in adding the feature just shout and I can check it works with master.

[RyanCavanaugh]

@jack-williams please revive, I'm back from leave now and am trying to get the PR backlog moving again. A fresh rebase with master would be great

[jack-williams]

@RyanCavanaugh done!

[RyanCavanaugh]

LGTM - great testcase btw. Will merge on Tuesday after the release branch finalizes

[jack-williams]

Great, and thank you!

[RyanCavanaugh]

Technically a breaking change, better bump the major version of TS 🙄

declare const u: unknown;
if (u === 0) {
    let m = u;
    // Now an error
    m = "";
}

(don't actually change anything)