Use submodule for VisualEditor core

* Add lib/ve.
* Remove modules provided by VisualEditor core.
* Update paths.
* Remove VisualEditor core specific things from Gruntfile.
* Remove entries from external.js already in lib/ve/.docs/external.js.
* Implement build script to compose a jsduck index that includes
  all of VisualEditor core. Right now it includes VisualEditor's
  index as-is, and because those patterns (e.g. ve.dm.*Node) will
  also MW subclasses this means our category page will include
  MW classes under "VisualEditor (core)" (before the repo split,
  this was intentional).
  This is inevitable unless we list every class individually, or
  (the plan) until we move the MW subclasses out of the ve.*
  namespace and into mw.* somewhere.

Bug: 45342
Change-Id: Iff45cd555430634c9fc341fd3b177c1e0625300b