ci: Enforce SCSS/CSS rules - add stylelint workflow

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

.github/workflows/lint-stylelint.yml

@@ -0,0 +1,51 @@
+# This workflow is provided via the organization template repository
+#
+# https://github.com/nextcloud/.github
+# https://docs.github.com/en/actions/learn-github-actions/sharing-workflows-with-your-organization
+#
+# SPDX-FileCopyrightText: 2021-2024 Nextcloud GmbH and Nextcloud contributors
+# SPDX-License-Identifier: MIT
+
+name: Lint stylelint
+
+on: pull_request
+
+permissions:
+  contents: read
+
+concurrency:
+  group: lint-stylelint-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    name: stylelint
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871 # v4.2.1
+
+      - name: Read package.json node and npm engines version
+        uses: skjnldsv/read-package-engines-version-actions@06d6baf7d8f41934ab630e97d9e6c0bc9c9ac5e4 # v3
+        id: versions
+        with:
+          fallbackNode: '^20'
+          fallbackNpm: '^10'
+
+      - name: Set up node ${{ steps.versions.outputs.nodeVersion }}
+        uses: actions/setup-node@0a44ba7841725637a19e28fa30b79a866c81b0a6 # v4.0.4
+        with:
+          node-version: ${{ steps.versions.outputs.nodeVersion }}
+
+      - name: Set up npm ${{ steps.versions.outputs.npmVersion }}
+        run: npm i -g 'npm@${{ steps.versions.outputs.npmVersion }}'
+
+      - name: Install dependencies
+        env:
+          CYPRESS_INSTALL_BINARY: 0
+        run: npm ci
+
+      - name: Lint
+        run: npm run stylelint