.github/workflows: add webclient workflow

Add workflow to run yarn lint/test/format-check against the web
client on pull requests.

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

.github/workflows/webclient.yml

@@ -0,0 +1,40 @@
+name: webclient
+on:
+  workflow_dispatch:
+  # For now, only run on requests, not the main branches.
+  pull_request:
+    branches:
+      - "*"
+    paths:
+      - "client/web/**"
+      - ".github/workflows/webclient.yml"
+      - "!**.md"
+  # TODO(soniaappasamy): enable for main branch after an initial waiting period.
+  #push:
+  #  branches:
+  #    - main
+
+concurrency:
+  group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  webclient:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+      - name: Install deps
+        run: ./tool/yarn --cwd client/web
+      - name: Run lint
+        run: ./tool/yarn --cwd client/web run --silent lint
+      - name: Run test
+        run: ./tool/yarn --cwd client/web run --silent test
+      - name: Run formatter check
+        run: |
+          ./tool/yarn --cwd client/web run --silent format-check || ( \
+            echo "Run this command on your local device to fix the error:" && \
+            echo "" && \
+            echo "  ./tool/yarn --cwd client/web format" && \
+            echo "" && exit 1)