Create Github Workflow script to automatically populate changelog with latest commit

Once we have templates for PRs, we will want a script that will, upon PR merge, prepend the commit message for the PR to the ChangeLog.

A couple nuances that @nusbaume and @cacraigucar pointed out:
- The script should have the ability to go back multiple commits to handle the initial changelog population
- The script will probably have to be allowed to force push to update the changelog
- Also, the git log will, as a result of this action, have an "updating changelog" commit after ever code commit

Start by reviewing the repository's existing GitHub workflow configuration and ChangeLog, since the issue does not name specific files or tests. Determine how merged pull requests and multiple prior commits should be selected, then define how the workflow updates and force-pushes the changelog. Done means the workflow can populate the initial changelog and update it after later merges without ambiguity.

Written by the indexing model from the issue text.