Added PHP language support

[JoshuaKGoldberg]

Fixes #102. The following major language changes are required to support PHP:

• Parameters and variables must have a $ at the beginning of their name when output in PHP - so a variable command must be used to refer to them (adding a parameter equivalent seems unnecessary for now)
• Sets may only contain numbers or strings (😢), as PHP arrays can only have number or string keys, and the potential SPL SplObjectStorage class and spl_object_hash function don't support those number or string literals
  • A later extension could add the concept of an "object" set and rename this one to "literal", to differentiate the two...

[JoshuaKGoldberg]

Per #102, PHP support will not be happening.