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/TestUtils.test.ts

@@ -269,6 +269,9 @@ export class MockInputHandlingTerminal implements IInputHandlingTerminal {
   addDisposableListener(type: string, handler: XtermListener): IDisposable {
     throw new Error('Method not implemented.');
   }
+  tabSet(): void {
+    throw new Error('Method not implemented.');
+  }
 }
 
 export class MockBuffer implements IBuffer {