Previously, the client would decide when to ask the server for
completions, and it would only do so for identifiers that had spans
(determined via the highlighter!).
Swap this around and make the server decide if it wants to complete
something.
This commit also adds a CompletionKind (which only has one value:
Identifier), to work with other kinds of completions as well.
This commit moves all the logic that deals with the language server
(from HackStudio) into a LanguageClient class, provides some functions
to make constructing them easier, and makes all language servers use a
singular IPC definition.
Also fixes the FIXME about making the autocompletion async.
This makes adding language servers in the future significantly less
duplicate-y, and significantly easier :^)
The language server keeps track of the content of currently edited
files by receiving updates about edit actions.
Also, C++ autocompletion is no longer tied to HackStudio itself and
moved to be part of the language server.
