[RPC][class_expand] Introduce new RPC command: Class Expand

[tkotosz]

In the documentation Class Expand mentioned as a feature but actually this command does exists. (RPC documentation is correctly not mentioning it)

I have checked the VIM plugin to see how it handles this class expand: It seem it does the following thing manually:

1. calls class_search RPC helper command (not user facing)
2. processes the returned return_choice "editor action" manually to apply the actual class expand action

I have skipped the implementation of this in the sublime plugin since I think this probably should be handled on the phpactor side by introducing a first-class command for class-expand.