Move Shared interfaces into a new typings file

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

src/handlers/Clipboard.ts

@@ -3,7 +3,7 @@
  * @license MIT
  */
 
-import { ITerminal, ISelectionManager } from '../Interfaces';
+import { ITerminal, ISelectionManager } from '../../typings/xterm-internal';
 
 interface IWindow extends Window {
   clipboardData?: {