Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

.github/workflows/build.yml

@@ -152,6 +152,8 @@ jobs:
           cache-dependency-path: frontend/package-lock.json
       - name: Install frontend dependencies
         run: cd frontend && npm ci
+      - name: Type-check frontend
+        run: cd frontend && npm run prebuild && npm run typecheck:all
       - name: Lint frontend
         run: cd frontend && npm run lint
       - name: Build frontend