Add support for FriendsOfPHP/PHP-CS-Fixer

[spaceemotion]

trafficstars

I've tried to implement support for FriendsOfPHP/PHP-CS-Fixer, but there are a few issues that prevent me from completing it:

• No real ability to specify files to look through (there's https://github.com/FriendsOfPHP/PHP-CS-Fixer/issues/4279 but it doesn't work without a config)
• No detailed output. It only shows me a list of files that are invalid, but no line numbers or anything.

[keradus]

• you may provide multiple files, eg https://github.com/FriendsOfPHP/PHP-CS-Fixer/issues/4279#issuecomment-820615223 (specifying a config shouldn't be an issue)
• use the schema, like checkfile, to receive such details: https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/2.18/doc/report-schema/checkstyle.xsd

[spaceemotion]

@keradus Thanks a lot for the followup! Will see what I can do....