Create a workflow that compares a PR with the build server

[desrosj]

When a pull request modifies files that affect the build output, it is currently difficult to know whether the resulting build directory would differ from what the build server currently has committed.

It would be helpful to have a workflow that compares the contents of the build directory within a pull request with WordPress/WordPress to make sure that there are no unexpected changes.

This changeset adds a new CI job to the "Check Built Files (PRs)" workflow that runs npm run build on the pull request's code and compares the resulting build directory against the corresponding branch of ​WordPress/WordPress.

The results are then posted as a pull request comment so that committers can review any differences without needing to check out the branch locally.

Trac ticket: Core-64878

## Use of AI Tools

Claude was used to create the first draft of this PR.

I think this is ready to try. I've tested it in ​https://github.com/desrosj/wordpress-develop/pull/505 while updating npm devDependencies and it seems to work well! The unexpected files in that list are due to how the build server does not clean up after itself every time and built files persist unless explicitly removed.

[desrosj]

While this would have been helpful to debug some of the unexpected changes to the built package during this cycle, I did not have time to wrap this up. It's not critical for 7.0, so punting.