Allow augmenting the global scope with new names

[yortus]

It would be great if TypeScript could model situations where new names are added to the global scope.

This was described by a team member as a desirable feature that could definitely be enabled in #7015 (comment). However that issue is closed, and I can't find any other open issue tracking the suggestion, so I opened this one.

A basic example:

// File: foo.ts

// Augment the global scope to add foo
declare global {
    var foo: Function;  // ERROR: Cannot introduce new top level names
}

// Implement foo
var fooImpl = () => {/***/};
foo = fooImpl;  // OK - compiler 'sees' global foo here
export default fooImpl;

// Reference foo
foo();          // OK - compiler 'sees' global foo here

Currently this produces a compiler error as shown above. Interestingly, the global foo is recognised at the two other sites where it is referenced in the example. So it appears this would already work just if the error message was suppressed.

[yortus]

Fixed by #8104?

[basarat]

Fixed by #8104?

@yortus indeed. Just tested and it works out of the box 🌹

[yortus]

Sweet! Thanks for checking @basarat