ci: Lint PR titles for Conventional Commits

PR titles become the commit message on main when squash-merging,
and release prep now runs on every push to main, so titles need
to follow the Conventional Commits format before merge. Copied
from the tailwind_merge repo.

Author: gjtorikian

.github/workflows/lint_pr_title.yml

@@ -0,0 +1,20 @@
+name: Lint PR Title
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+
+permissions:
+  pull-requests: read
+
+jobs:
+  main:
+    name: Validate PR title
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@48f256284bd46cdaab1048c3721360e808335d50 # v6.1.1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}