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/renderer/SelectionRenderLayer.ts

@@ -3,10 +3,8 @@
  * @license MIT
  */
 
-import { IBuffer, ICharMeasure, ITerminal } from '../Types';
-import { CHAR_DATA_ATTR_INDEX } from '../Buffer';
-import { GridCache } from './GridCache';
-import { FLAGS, IColorSet, IRenderDimensions } from './Types';
+import { ITerminal } from '../Types';
+import { IColorSet, IRenderDimensions } from './Types';
 import { BaseRenderLayer } from './BaseRenderLayer';
 
 export class SelectionRenderLayer extends BaseRenderLayer {