Add noUnusedLocals tsconfig flag, fix issues

Two parts were commented out:
- _writeStopped: This is because _xoffSentToCatchUp references this variable
  and the feature is sort of half implemented/disabled.
- _clearChar: This is because some comments still reference this, it's part
  of the work needed to do better line redrawing.

src/utils/CharMeasure.ts

@@ -3,7 +3,7 @@
  * @license MIT
  */
 
-import { ICharMeasure, ITerminal, ITerminalOptions } from '../Types';
+import { ICharMeasure, ITerminalOptions } from '../Types';
 import { EventEmitter } from '../EventEmitter';
 
 /**