ci: introduce release-please (#427)

Adopt googleapis/release-please-action for automated release PRs based
on Conventional Commits, mirroring the gohan setup.

- .github/workflows/release-please.yml: run on push to main
- .release-please-config.json: go release type, v-prefixed tags,
  conventional-commit changelog sections
- .release-please-manifest.json: seed at current release (8.4.0)

When commits land on main, release-please opens / updates a release PR
that bumps CHANGELOG.md and (on merge) creates the next vX.Y.Z tag,
which in turn triggers the existing GoReleaser workflow.

Author: bmf-san

.github/workflows/release-please.yml

@@ -0,0 +1,21 @@
+name: release-please
+
+on:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  release-please:
+    name: Run release-please
+    runs-on: ubuntu-latest
+    steps:
+      - name: release-please
+        uses: googleapis/release-please-action@v4
+        with:
+          config-file: .release-please-config.json
+          manifest-file: .release-please-manifest.json

.release-please-config.json

@@ -0,0 +1,25 @@
+{
+  "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
+  "release-type": "go",
+  "include-component-in-tag": false,
+  "include-v-in-tag": true,
+  "bump-minor-pre-major": false,
+  "bump-patch-for-minor-pre-major": false,
+  "packages": {
+    ".": {
+      "package-name": "ggc",
+      "changelog-sections": [
+        { "type": "feat", "section": "Features" },
+        { "type": "fix", "section": "Bug Fixes" },
+        { "type": "perf", "section": "Performance Improvements" },
+        { "type": "revert", "section": "Reverts" },
+        { "type": "refactor", "section": "Refactors" },
+        { "type": "docs", "section": "Documentation" },
+        { "type": "test", "section": "Tests" },
+        { "type": "build", "section": "Build System" },
+        { "type": "ci", "section": "Continuous Integration" },
+        { "type": "chore", "section": "Miscellaneous", "hidden": false }
+      ]
+    }
+  }
+}

.release-please-manifest.json

@@ -0,0 +1,3 @@
+{
+  ".": "8.4.0"
+}