Add a github actions workflow to create a release PR (#1437)

The action runs the existing create_release.py script in a new mode
created for github actions. Rather than using local git commands, it
uses a public action for creating a PR in the repository. The action can
be triggered from the website or via the CLI tool, for example

gh workflow run create-release.yml -R emscripten-core/emsdk -F
lto-sha=abc123 -F nonlto-sha=234567

.github/workflows/create-release.yml

@@ -0,0 +1,27 @@
+name: Create Release PR
+
+on:
+  workflow_dispatch:
+    inputs:
+      lto-sha:
+        required: true
+        type: string
+      nonlto-sha:
+        required: true
+        type: string
+
+jobs:
+  create-release-pr:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+      - name: Run create_release.py
+        run: python3 scripts/create_release.py ${{ inputs.lto-sha }} ${{ inputs.nonlto-sha }} --action
+      - name: Create PR
+        id: cpr
+        uses: peter-evans/create-pull-request@v6
+        with:
+          title: Release ${{ env.RELEASE_VERSION }}
+          commit-message: Release ${{ env.RELEASE_VERSION }}
+          delete-branch: true