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/SelectionModel.test.ts

@@ -18,9 +18,6 @@ class TestSelectionModel extends SelectionModel {
 }
 
 describe('SelectionManager', () => {
-  let window: Window;
-  let document: Document;
-
   let terminal: ITerminal;
   let model: TestSelectionModel;