Support completion for console and watches.

Add omnifunc for prompt buffers

This synchronous completion can be used with any completion system
including built-in CTRL-X CTRL-O.

The filetype of the prompt buffers is set to VimspectorPrompt so that it
can be identified by completion systems. For example, this works well
with YCM:

let g:ycm_semantic_triggers =  {
  \   'VimspectorPrompt': [ '.', '->', ':', '<' ]
  \ }

python3/vimspector/debug_session.py

@@ -554,8 +554,7 @@ class DebugSession( object ):
     # TODO:
     #  - start / length
     #  - sortText
-    return [ i.get( 'text' ) or i[ 'label' ]
-             for i in response[ 'body' ][ 'targets' ] ]
+    return response[ 'body' ][ 'targets' ]
 
 
   def _SetUpUI( self ):