first pass at compat.php checker

[aaronjorbin]

Trac ticket: https://core.trac.wordpress.org/ticket/61694

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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.

[github-actions]

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[peterwilsoncc]

Is it possible to do this as a custom sniff rather than a separate action?

That will allow the errors to display in IDEs rather than relying on contributors pushing PRs which may not always happen. There is some prior art for custom sniffs in the Gutenberg Repo https://github.com/WordPress/gutenberg/tree/trunk/test/php/gutenberg-coding-standards

I'm not sure it's worth putting in the main WPCS repo (others may have a different view)