Enhance readme CLI command to support standalone plugins

[felixarntz]

Summary

Follow up to #1064: This PR enhances the npm run readme command to support standalone plugins. Given the changelog command was already updated, the only necessary change was to dynamically identify which readme.txt file to update, as before it would always update the root readme.txt file.

Relevant technical choices

• A path option was added, however that's purely for flexibility and is optional.
• By default, the path will automatically be detected via the milestone option.
  • Going forward, milestones should be named plugin-slug v.e.r.s.i.o.n so that this script can detect the relevant plugin automatically.
  • This will most likely be beneficial for other future workflow automation enhancements too.

You can test this by e.g. running:

• npm run readme -- -m "performance-lab 3.0.0"
• npm run readme -- -m "speculation-rules 1.1.0"

Don't commit the changes though, it's just for testing, particularly as both changelogs at this point contain a few manually added entries that are worth keeping.

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[mukeshpanchal27]

Thanks for the PR @felixarntz. LGTM!