Move Shared interfaces into a new typings file

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

src/utils/CharMeasure.ts

@@ -3,8 +3,8 @@
  * @license MIT
  */
 
+import { ICharMeasure, ITerminal, ITerminalOptions } from '../../typings/xterm-internal';
 import { EventEmitter } from '../EventEmitter';
-import { ICharMeasure, ITerminal, ITerminalOptions } from '../Interfaces';
 
 /**
  * Utility class that measures the size of a character. Measurements are done in