GHA CI: Backport latest changes (#15361)

By having unified github workflows, the building cache would be utilized
more efficiently as the total cache size will be smaller (no more vcpkg
caching on macOS CI) and will stop thrashing the build cache (large
vcpkg cache evicts other smaller cache).

Relevant PRs:
#15321
#15340
#15342
#15355

.github/workflows/ci_webui.yaml

@@ -0,0 +1,32 @@
+name: CI - WebUI
+
+on: [pull_request, push]
+
+jobs:
+  ci:
+    name: Check
+    runs-on: ubuntu-latest
+
+    defaults:
+      run:
+        working-directory: src/webui/www
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Setup nodejs
+        uses: actions/setup-node@v2
+        with:
+          node-version: 'lts/*'
+
+      - name: Install tools
+        run: npm install
+
+      - name: Lint code
+        run: npm run lint
+
+      - name: Format code
+        run: |
+          npm run format
+          git diff --exit-code