Setup linter (#12)

Fixes basic issues. For now, we're still allowing `any`, but it'd be
nice to move away from it.

.github/workflows/ci.yml

@@ -35,6 +35,9 @@ jobs:
       - name: Check formatting
         run: npm run format:check
 
+      - name: Run linter
+        run: npm run lint
+
       - name: Build
         run: npm run build