Update types for extensibility

[dfreeman]

This PR proposes a few changes to the types in order to provide tools like Glint the hooks necessary to augment them with additional information for template validation. In particular, it:

• creates dedicated interfaces for the type of on and fn using the same approach as https://github.com/DefinitelyTyped/DefinitelyTyped/pull/58606
• creates interfaces for the return types of hbs and helper(), following the same approach as https://github.com/DefinitelyTyped/DefinitelyTyped/pull/59714

It also adds a very basic type test suite using expect-type

Ping (low urgency) @chriskrycho for a sanity-check 🙂

[chadhietala]

@chriskrycho @dfreeman Just wondering whats left to do here as it seems like this is the last PR to remove the glimmerx re-exports in glint.

[dfreeman]

I think all that's needed here is a merge and release from a maintainer. Once that's done we'll need to make some corresponding changes in @glint/environment-glimmerx and publish a new release of that as well.

[chadhietala]

I merged this but looks like the tests are failing on master, not just the ember ones that I think were failing before this.

[dfreeman]

Looks like an outdated Babel version. I'll see if I can get a PR together to get tests green more generally

[dfreeman]

@chadhietala things should be ✅ across the board in https://github.com/glimmerjs/glimmer-experimental/pull/154

[chadhietala]

@dfreeman 0.6.7 is released with these changes

[dfreeman]

@chadhietala Thanks! Hopefully we should be able to get an updated @glint/environment-glimmerx out in the next week or two