registration: Let extensions add PHP version requirements

While MediaWiki Core already sets requirements for PHP versions, it should be
possible for extensions to tighten these requirements. This mirrors the PHP
parameter of extension infoboxes as well.

This change introduces a new 'platform' key (in addition to 'MediaWiki', 'skins'
and 'extensions', where non-MediaWiki software requirements will be listed
in the future, starting with a PHP version constraint. Further keys are
supposed to be added to allow setting constraints against php extensions
and other abilities of the platform.

Bug: T197535
Change-Id: I6744cc0be2363b603331af9dc860eb8603a1a89a