Prepare release automation

Adds automation to prepare a classic buildpack release. See heroku/languages-github-actions#183

.github/workflows/prepare_release.yml

@@ -0,0 +1,16 @@
+name: Prepare Buildpack Release
+
+on:
+  workflow_dispatch:
+  # TODO: remove the pull_request trigger before merge, it's just for testing
+  pull_request:
+
+jobs:
+  prepare-release:
+    uses: heroku/languages-github-actions/.github/workflows/_classic-buildpack-prepare-release.yml@prepare_release_for_classic_buildpacks
+    with:
+      app_id: ${{ vars.GH_APP_ID }}
+      buildpack_name: heroku/nodejs
+      unreleased_header_text: main
+    secrets:
+      app_private_key: ${{ secrets.GH_APP_PRIVATE_KEY }}