registration: Allow to require environment abilities

registration: Allow to require environment abilities

This patch adds the possibility for extensions and skins to require
certain environment abiltites that are not necessarily PHP extensions.

For now, the only ability introduced is the ability to shell out, but
the processing and testing is written in a more general way to allow
users to add more abilties later on by just changing getAbilities().

In theory, this allows using VersionChecker to check for random
abilities if they are specified in the constructor, as it is comletely
environment agnostic and not really bound to just be used for checking
extension compatibility.

Furthermore, it is possible to specify custom error messages for each of
these abilities in the constructor. Other parts of MediaWiki may use
these features to check for requirements while working with totally
different abilities.

Bug: T212472
Change-Id: Iff8512530b08ef509e7ac0b6ed8fe9578ef3e2a1