Add prepare changelog workflow to automatically propose what should be in sync for us

[NullVoxPopuli]

Supersedes #1360

Here is a demo of it running on my fork: https://github.com/NullVoxPopuli/embroider/actions/runs/4746443242/jobs/8430099950

And here is the PR it created: NullVoxPopuli#1

And here is the PR for resolving all the TODOs, and verifying impact / affected packages: #1401

I still need to add support for an .changesetignore~style file for ignoring certain changes (or ignoring aggregated things)

But the proposed workflow here is less manual than before:

1. wait for one of these PRs to be opened
2. go through the TODOs
3. create another PR with the TODOs taken care of
4. merge

• this can happen async with other work (and by anyone who wants to review merged PRs and assess impact, or create more detailed changelog entries) -- the main thing is that the automation gets the process started.
• this can also happen incrementally, a PR that submits these changeset entries does not need to contain all of them

For an example of how detailed the changelog entries can get: https://github.com/NullVoxPopuli/ember-resources/blob/main/.changeset/gold-weeks-destroy.md?plain=1
And how it looks rendered in the changelog: https://github.com/NullVoxPopuli/ember-resources/blob/main/ember-resources/CHANGELOG.md#major-changes-2