Support running keymap-editor locally

[MattSturgeon]

Now that keymap-drawer can be run using nix run .#draw, it'd be handy to also have a nix run .#edit that runs keymap-editor locally

• [ ] Build a nix package using buildNpmPackage
• [ ] Build a wrapper script that launches the server & opens the browser
• [ ] Ensure APP_BASE_URL=http://localhost:3000 is exported when running