Move Shared interfaces into a new typings file

This will enable sharing the internal interface with addons (and catching
breakages early)

src/SelectionModel.test.ts

@@ -4,7 +4,7 @@
  */
 
 import { assert } from 'chai';
-import { ITerminal } from './Interfaces';
+import { ITerminal } from '../typings/xterm-internal';
 import { SelectionModel } from './SelectionModel';
 import { BufferSet } from './BufferSet';
 import { MockTerminal } from './utils/TestUtils.test';