Add TS type definition for hbs import

[josemarluedke]

Adds simple type definition for the hbs import import { hbs } from 'ember-template-imports';.

Helps with #14.

[chriskrycho]

👍🏼 to this as a good stopgap here.

[chriskrycho]

Hmm. The TemplateFactory definition here won't actually do anything; it's just an empty interface. We need to do one of:

• importing the actual type from Ember's types and using that so we can guarantee interop (recommended)
• making it return unknown, which will require casts for folks to use it (not recommended)

[chriskrycho]

As we start to move toward stabilizing <template>, I'm doing some cleanup—I welcome input from @dfreeman and @jamescdavis here on the TS front, and from other folks on Framework on the more general front:

My very strong intuition here is that we should not ship a specific type for this hbs, and indeed move to deprecate it.

[dfreeman]

If you're using Glint with ember-template-imports today, I believe we provide a declaration for hbs there (and if you're not using Glint, you're going to get a bunch of spurious "unused symbol" errors from vanilla TS).

As we move toward our next breaking release of Glint, though, I'm inclined to agree with @chriskrycho and say we should probably drop that and discourage the use of hbs since <template> has been officially adopted as the path forward.