[ci] Automate adding git diff comment to release PRs

[haworku]

Is your feature request related to a problem? Please describe. A release PR in this library is a simply a commit to main with the updated changelog and package.json version. This is nice but it does present a challenge for code review because its not always apparent to new reviewers what code is being released.

To make sure reviewers can check the actual code change between a previous released version and the current release, we have a habit of manually adding a comment (or line in the PR description) with a link to the git diff between versions.

Example: https://github.com/trussworks/react-uswds/pull/1891#issuecomment-1023619153

Describe the solution you'd like

Automate this. A bot adds the comment with the git diff between last released version and current version ready to be released.

Describe alternatives you've considered

• do nothing. this means with rotating maintainers and reviewers it might be easier for folks releasing code to miss what code change is actually being released on npm with a release.

[werdnanoslen]

@haworku do you know if release-please can do this?