Language servers will now receive an open file instead of just its path. This
means the language servers no longer need to access the filesystem to open the
file themselves.
The C++ language server now has no filesystem access whatsoever (although we
might need to relax this in the future if it learns to complete #include paths),
while the Shell language server can read /etc/passwd (it wants that in order to
get the user's home directory) and browse (but not read!) the whole file system
tree for completing paths.
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 :^)
