Introduce environments

[dfreeman]

Overview

Up to now, @glint/template contained a kind of hodge-podge of knowledge about keywords and globals available in different contexts (GlimmerX vs Glimmer.js vs Ember etc), while @glint/cli and @glint/tsserver-plugin were both hard-coded to look for @glimmerx/component in a module to determine whether to attempt template transformation.

This PR introduces the notion of an environment that a given project is targeting. An environment specifies information used in transforming templates (such as where to look for tag imports from) as well as exposing signatures for the available keywords/globals and determining how userspace entities (like components and custom helpers) interact with the type system.

Down the line, if we consider things like supporting current Ember idioms, environments may also influence things like whether templates can "see" in-scope identifiers or not.

Notes

• A .glintrc file is now required in order to determine the active environment for a project
• Code emitted @glint/transform no longer references @glint/template, but rather the configured environment (which likely re-exports a number of types from @glint/template)
• The tests for @glint/cli and @glint/tsserver-plugin now stage their test apps under test-packages/ephemeral rather than in system tmp, as package resolution starts to get wonky otherwise (PnP would be the 'real' solution for this... someday 😭)
• A refactoring of @glint/template (its exports, not its internals) is likely coming, but not something that needs to be tangled up with this
• Some of the docs in the README are no longer quite accurate with this change, but I have a larger refactoring there (in particular, breaking it out into more consumable chunks) brewing, so I'm leaving it as-is for now