add support for file_info rpc action

[kermorgant]

add support for phpactor's file_info rpc action which returns class FQN, name and namespace about currently open file (see PR bringing this into phpactor)

[kermorgant]

Not sure the phpactor-insert-namespace really belongs here. Opinions ?

[zonuexe]

LGTM

[ejmr]

Not sure the phpactor-insert-namespace really belongs here. Opinions?

Since the subject of the code is relevant file info (apparently), the namespace for the given code in a file seems appropriately on target. Unless I am misunderstanding the purpose of that piece of code.

[kermorgant]

phpactor-file-information refers to the file_info rpc action, whereas php-insert-namespace is a trivial function using it.

In other words, I wonder if at some point, we should differentiate the functions aimed at bringing phpactor.el to emacs from other built upon them.