build: add justfile

This makes it easy to spin up a development server that will rebuild on
file changes.

justfile

···
+[parallel]
+serve: serve-js serve-rs
+
+[working-directory: 'frontend']
+serve-js:
+	npm start
+
+[working-directory: 'server']
+serve-rs $RUST_LOG=env('RUST_LOG', 'debug'):
+	watchexec --restart --ignore "target/**" cargo run