emsdk

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