ci(github): compliance test for conventional commits

This adds a GitHub action workflow that runs commitizen's `cz check`
on the revision range of a PR. The workflow errors when any commit
message is not compliant with the chosen convention, which is the
`cz_customize` configuration in `pyproject.toml` at this point.

Refs: #10

.github/workflows/conventional-commits.yml

@@ -0,0 +1,24 @@
+name: Conventional commits
+
+on: pull_request
+
+jobs:
+  check-messages:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: 3.11
+          architecture: x64
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          # we need all the history to be able to resolve revision ranges properly
+          fetch-depth: 0
+      - name: Install commitizen
+        run: python -m pip install commitizen
+      - name: Run commit message checks
+        run: |
+          echo cz check --rev-range ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }}
+          cz check --rev-range ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }}