.github: check-metadata: new workflow

Add a GitHub Actions workflow to run the maintenance script
"check-metadata.sh" on pushes to branch 'main' and on pull requests
which target branch 'main'.

I'm not sure if `concurrency:` is needed.

I used the following GitHub Actions files as inspiration:

- For overall structure:
  - https://github.com/curl/curl/blob/c39585d9b7ef3cbfc1380812dec60e7b275b6af3/.github/workflows/linkcheck.yml
- For job naming and Bash script launch:
  - https://github.com/rybak/intellij-copy-commit-reference/blob/47a64c7408d854f73fa59a053d9346c27122bafe/.github/workflows/build.yml

.github/workflows/check-metadata.yml

@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: MIT
+
+name: Check metadata
+
+on:
+  push:
+    branches:
+    - main
+  pull_requests:
+    branches:
+    - main
+    paths:
+      - '*.user.js'
+      - '*.user.css'
+      - 'maintenance/check-metadata.sh'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+permissions: {}
+
+jobs:
+  check:
+    name: Check metadata
+    runs-on: ubuntu-latest
+    steps:
+    - name: Fetch source code
+      uses: actions/checkout@v4
+
+    - name: Do check metadata
+      run:
+        ./maintenance/check-metadata.sh

maintenance/check-metadata.sh

@@ -1,5 +1,11 @@
 #!/bin/bash
 
+# Check metadata of userscripts and userstyles
+#
+# Used by GitHub Actions. Can be used locally:
+#
+#	$ ./maintenance/check-metadata.sh
+
 set -e
 set -u