PHPCS: various minor tweaks

[jrfnl]

Proposed Changes

• Add the PHPCS custom ruleset to the export-ignore list in .gitattributes.
• Add the files which can be used by individual devs to overrule the project PHPCS ruleset to the .gitignore file.
• Strip down the file paths PHPCS shows in reports to the path relative to the project root.

[frenck]

Thanks! 👍