Introduce @glint/environment-ember-loose

[dfreeman]

This PR begins to address #23. While this isn't yet a complete implementation, it provides a core we can use to develop against while building out support for standalone template files elsewhere in glint.

Points of interest:

• Environments can now specify information about how to handle standalone .hbs files, including what types module applies to them and how to map between templates and their backing scripts
• An initial pass at mapping templates to backing scripts for this environment supports co-located and pod-based components, as those are trivial to map.
• @glint/environment-ember-loose includes signatures for Glimmer components, Ember components and helpers, and ember-modifier modifiers. It doesn't handle controllers/routes yet (there are some potentially interesting design questions there), but those are an important aspect of today's Ember programming model, so eventually that needs to be accounted for.
• Class-based helpers currently can't implement compute in a typesafe way without clashing with the upstream definition in @types/ember__component. This is similar to the problem we have in Figure out the shape of component type parameters #25 where we want to capture more type information than the base class is currently interested in. One short-term option might be for the environment to include re-exports of the upstream base classes cast to the appropriate types, but for now we just have @ts-expect-error
• A handful of Ember-specific intrinsics are currently set up (action, each-in, link-to, log), as well as the core keywords defined in @glint/template, but many more remain to be added.