Add VS Code extension

[dfreeman]

This PR begins with the @glint/vscode extension that @jamescdavis set up in #40 and then layers on a few tweaks and gets a basic integration test set up. Unit tests don't buy us much here since essentially all of the code is just glue between Code and the language server.

Notes:

• This diff is a bit more consumable with ?w=1/"hide whitespace changes", as there are a couple chunks where just indentation changed
• There's a bit of boilerplate under __tests__/support to handle launching Jest within Code's test harness. There are some folks experimenting with packaging something up for more easily getting that working (see Add jest-runner microsoft/vscode-test#59 and linked issues), but nothing's quite there yet and for now this suits our purposes.
• This now supports multi-root workspaces by launching a language server per root (which is the recommended approach. We still don't support multiple TypeScript/Glint projects within a root, which is valuable for monorepos and/or composite projects, and is something we should likely investigate further.
• The repo root now contains .vscode/launch.json configuration for launching a debug Code instance with the under-development extension