Add workflow to compare PR with the build server.

[desrosj]

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.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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Props desrosj.

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[desrosj]

I think this is ready to try. I've tested it in desrosj#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.