chore: Prevent PRs targetting the master branch (#650)

* chore: Prevent PRs targetting the master branch

* chore: Escape branch names

* chore: Checkout

* chore: Don't run on reopen (as you need to reopen to change the base branch)

.github/workflows/pr-base-enforcement.yml

@@ -0,0 +1,21 @@
+name: Prevent PRs targetting master
+
+on:
+  pull_request:
+    types: [opened, edited]
+    branches:
+      - master
+
+jobs:
+  prevent-pr-targetting-master:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332
+      - name: Post comment to update base branch
+        run: gh pr comment ${{ github.event.pull_request.number }} --body "⚠️ Pull requests targetting the \`master\` branch are not allowed. Please update the base branch to \`next\`."
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Close PR
+        run: gh pr close ${{ github.event.pull_request.number }} --repo ${{ github.repository }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}