Add support for "go to definition" using haxe 3.2 new --display @position feature

This adds `ctrl+]` as a default shortcut for a new `haxe_find_definition` command that uses new haxe editor support features currently found in Haxe/git and soon(ish) in Haxe 3.2.

I had to hack the `HaxeComplete.run_haxe` command a bit to make it work, I guess it should be properly refactored to support new compiler features (toplevel completion, usage), but for now I think that change is quite minimal so it's okay.

Haxe supports finding positions of fields and local vars and to some extent of types.