ci: add release-please for automated release PRs (#71)

Accumulates conventional commits into a Release PR that bumps the
version and updates CHANGELOG.md. Nothing is tagged until a human
merges the Release PR.

Co-authored-by: Jasper <jasper@magnus919.com>

.github/workflows/release-please.yml

@@ -0,0 +1,17 @@
+name: release-please
+
+on:
+  push:
+    branches: [main]
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  release-please:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: googleapis/release-please-action@v4
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}