[WIP] autocomplete-plus support

[ssfrr]

This is a first cut at autocomplete support. It assumes you're using
supercolliderjs with my recent socket PR.

Here it pulls the list of class names and documentation summaries right after
booting the server, and queries for class methods on-the-fly.

I agree with your comment elsewhere that loading the class info from a file
would be nice so we can still auto-complete if SCLang is crashing or otherwise
unavailable. The balance is basically making sure that the autocompletion is
up-to-date whenever possible, so if the user adds a Quark or custom class it
should show up after their next reboot. In general a reasonable fallback plan
seems like:

1. Use fresh data (from the last SCLang boot) whenever possible
2. If SCLang didn't boot successfully, use a cached copy from the last boot
3. If there are no cached copies, load from a default file that ships with the
   package with all the classes that ship with SC.

I'll test out whether getting all the class methods is fast enough to do it
up-front rather then at autocomplete-time.

Still more to do (this is somewhat aspirational, we'll see how far I get):

• handle inherited methods
• merge Class.prop and Class.prop_ methods with some
  readable/writable indication
• figure out cacheing for when SCLang is unavailable
• better feedback when you're in the middle of an argument list
  (maybe snippet support?)
• add variable support - if we can statically figure out the class of a
  variable it would be great to be able to autocomplete instance
  members/methods
• incorporate the regular text-based autocomplete so we also complete
  identifiers in the file.

I'll add a demo gif shortly. :)

[ssfrr]

I started this before those other PRs were merged, so I'll need to rebase.

Here's a gif: