Testing BuddyPress against its own/new PHP Code Sniffer (PHPCS).

The plugin is using its own custom PHP Code Sniffer (​https://github.com/buddypress/bp-coding-standards)

A non-blocking Github Action was introduced to test against this new code sniffer. Actual PHPCS fixes will be introduced in future commits.

Closes ​https://github.com/buddypress/buddypress/pull/21
See #7228