Resolver-like component/helper/modifier resolution

[dfreeman]

To support today's Ember idioms in advance of template imports and strict mode, we need a way for glint to resolve template entities that aren't explicitly statically in scope.

The resolution process in @glint/template was designed with this in mind, and a likely approach here is to have the Ember environment we create in #23 include a publicly-extensible type registry in the set of global values it exposes to glint.

To make this less of a pain (particularly for the initial setup), it would be great if we could automate the process of generating this registry.

[dfreeman]

This is handled in @glint/environment-ember-loose/types/registry. Codegen may still be something worth exploring at some point, but the core issue here has been addressed.