Automate changelog from release notes

[GaryJones]

Props to @johnbillion for the work he had done on https://github.com/johnbillion/user-switching that this work is very minorly tweaked from.

• The README.md contains a HTML comment as a placeholder.
• During the deploy workflow (which happens when a non-pre-release release is made from a tag), it connects to the GitHub API, and pulls down previous release notes, removes any pre-release entries, sorts them into order, and loops through to create a string of headings and release notes.
• This then replaces the HTML comment in the README.md file.

Now instead of maintaining release notes in a CHANGELOG file or in the README, AND also in the GitHub release notes, it's now a single location (GitHub release notes).

The changelog.js workflow script can also be called locally, and this will do a one-time replacement of the HTML comment (for testing or removal of this automation). It needs a GitHub personal access token, and then call:

TOKEN=... GITHUB_REPOSITORY='Automattic/ad-code-manager' node .github/workflows/changelog.js

[johnbillion]

@GaryJones I had planned on converting this into a standalone composite action so it's reusable across repos but haven't gotten around to it. So if you find yourself twiddling your thumbs...

[GaryJones]

@johnbillion I did wonder if that was on your radar. Would make sense to do that. Not something I've created before.