Move Shared interfaces into a new typings file

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

src/utils/CircularList.ts

@@ -4,7 +4,7 @@
  */
 
 import { EventEmitter } from '../EventEmitter';
-import { ICircularList } from '../Interfaces';
+import { ICircularList } from '../../typings/xterm-internal';
 
 /**
  * Represents a circular list; a list with a maximum size that wraps around when push is called,