Introduce LanguageMode wrapper for Ace modes as a foundation of our own modes

LanguageMode is the central point for all language-specific behavior associated with an EditSession. There is one LanguageMode instance per EditSession. LanguageMode has access to the EditSession and its TokenizedBuffer, and in reverse the EditSession, DisplayBuffer, and TokenizedBuffer also make use of LanguageMode. This is a bit incestuous, but I think it's okay because you can think of LanguageMode as a swappable strategy object that governs language-specific aspects of that constellation of objects.

src/app/edit-session.coffee

@@ -1,6 +1,7 @@
 Point = require 'point'
 Buffer = require 'buffer'
 Anchor = require 'anchor'
+LanguageMode = require 'language-mode'
 DisplayBuffer = require 'display-buffer'
 Cursor = require 'cursor'
 Selection = require 'selection'
@@ -22,6 +23,7 @@ class EditSession
 
   scrollTop: 0
   scrollLeft: 0
+  languageMode: null
   displayBuffer: null
   anchors: null
   anchorRanges: null
@@ -34,7 +36,8 @@ class EditSession
   constructor: ({@project, @buffer, @tabText, @autoIndent, @softTabs, @softWrap}) ->
     @id = @constructor.idCounter++
     @softTabs ?= true
-    @displayBuffer = new DisplayBuffer(@buffer, { @tabText })
+    @languageMode = new LanguageMode(this, @buffer.getExtension())
+    @displayBuffer = new DisplayBuffer(@buffer, { @languageMode, @tabText })
     @tokenizedBuffer = @displayBuffer.tokenizedBuffer
     @anchors = []
     @anchorRanges = []
@@ -242,17 +245,17 @@ class EditSession
     @displayBuffer.largestFoldStartingAtScreenRow(screenRow)
 
   indentationForRow: (row) ->
-    @tokenizedBuffer.indentationForRow(row)
+    @languageMode.indentationForRow(row)
 
   autoIndentTextAfterBufferPosition: (text, bufferPosition) ->
     return { text } unless @autoIndent
-    @tokenizedBuffer.autoIndentTextAfterBufferPosition(text, bufferPosition)
+    @languageMode.autoIndentTextAfterBufferPosition(text, bufferPosition)
 
   autoOutdentBufferRow: (bufferRow) ->
-    @tokenizedBuffer.autoOutdentBufferRow(bufferRow)
+    @languageMode.autoOutdentBufferRow(bufferRow)
 
   toggleLineCommentsInRange: (range) ->
-    @tokenizedBuffer.toggleLineCommentsInRange(range)
+    @languageMode.toggleLineCommentsInRange(range)
 
   mutateSelectedText: (fn) ->
     @transact => fn(selection) for selection in @getSelections()