release preview workflow

[NullVoxPopuli]

The goal is to, on every update to main, update or create a PR that previews what the release would look like if we were to release at the moment.

In the future, we could add more automation where, if that PR is merged, a release occurs.

Here is what the PR looks like: NullVoxPopuli#3
(You'll have to ignore the specific changes detected, I don't have all the tags on my fork)

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My tests: https://github.com/NullVoxPopuli/embroider/actions/workflows/release-plan-preview.yml

Example of error when PRs don't have labels:

• https://github.com/NullVoxPopuli/embroider/actions/runs/5047459971/jobs/9054358495

Example of success: https://github.com/NullVoxPopuli/embroider/actions/runs/5048803284/jobs/9057440106

Example commit (as screenshot, because these won't last forever):

[ef4]

Thanks, this is good. It would be nice if the output of explain-plan could go into the body of the pull request.