Meta: Add basic commit message linting for pull requests

This new check will ensure that all commit message lines are at most 72
characters long, as well as ensure the commit title conforms to the
"Category: Brief description of what's being changed" format.

.github/workflows/cmake.yml

@@ -231,8 +231,34 @@ jobs:
       timeout-minutes: 4
       if: ${{ matrix.with-fuzzers == 'NO_FUZZ' }}
 
+  lint_commits:
+    runs-on: ubuntu-20.04
+    if: always() && github.event_name == 'pull_request'
+
+    steps:
+    - name: Get PR Commits
+      id: 'get-pr-commits'
+      uses: tim-actions/get-pr-commits@55b867b9b28954e6f5c1a0fe2f729dc926c306d0
+      with:
+        token: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Check Line Length
+      uses: tim-actions/commit-message-checker-with-regex@v0.3.1
+      with:
+        commits: ${{ steps.get-pr-commits.outputs.commits }}
+        pattern: '^.{0,72}(\n.{0,72})*$'
+        error: 'Commit message lines are too long (maximum allowed is 72 characters)'
+
+    - name: Check subsystem
+      if: ${{ success() || failure() }}
+      uses: tim-actions/commit-message-checker-with-regex@v0.3.1
+      with:
+        commits: ${{ steps.get-pr-commits.outputs.commits }}
+        pattern: '^\S+?: .+'
+        error: 'Missing category in commit title (if this is a fix up of a previous commit, it should be squashed)'
+
   notify_irc:
-    needs: [build_and_test_serenity, build_and_test_lagom]
+    needs: [build_and_test_serenity, build_and_test_lagom, lint_commits]
     runs-on: ubuntu-20.04
     if: always() && github.repository == 'SerenityOS/serenity' && (github.event_name == 'pull_request' || (github.event_name == 'push' && github.ref == 'refs/heads/master'))