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.test.ts

@@ -4,7 +4,7 @@
  */
 
 import jsdom = require('jsdom');
-import { ICharMeasure, ITerminal } from '../Types';
+import { ICharMeasure } from '../Types';
 import { assert } from 'chai';
 import { CharMeasure } from './CharMeasure';