Support standalone template files in @glint/cli

[dfreeman]

Overview

This PR introduces support for typechecking separate template files from the CLI, as well as adding the required functionality to @glint/transform to support that.

Notes and Caveats

• If you look at 36ab4d8, I'd recommend doing it with ?w=1, as it involved restructuring some test modules
• This doesn't include support for template-only components, as there are still some design questions to be answered about how that works. (Is anything about that environment-specific? How are types for args/yields/etc specified?)
• This doesn't include any updates to the tsserver plugin (other than matching an API change in @glint/transform), which will require several additional steps to support

@jamescdavis 0b82ced is the commit that updates the CLI itself. The main things happening there are ensuring that watching a probable component script also sets up a watch for its potential template files, and that reading a probable component script also reads the corresponding template.