Add PR title lint workflow

Enforce conventional-commit PR titles using
amannn/action-semantic-pull-request.

Refs: #3

.github/workflows/pr-title-lint.yml

@@ -0,0 +1,41 @@
+name: PR Title Lint
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+      - reopened
+
+permissions:
+  pull-requests: read
+
+jobs:
+  lint-pr-title:
+    name: Validate PR title
+    runs-on: ubuntu-latest
+    steps:
+      - name: Lint PR title
+        uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            feat
+            fix
+            docs
+            style
+            refactor
+            perf
+            test
+            build
+            ci
+            chore
+            revert
+          requireScope: false
+          subjectPattern: ^(?![A-Z]).+$
+          subjectPatternError: |
+            The subject "{subject}" found in the pull request title "{title}"
+            didn't match the configured pattern. Please ensure that the subject
+            doesn't start with an uppercase character.