addCustomWheelEventHandler API

I opted for consistency with the key event handler over our more modern
approach, we can migrate them at the same time if we end up returning
a disposable.

See microsoft/vscode#76381

src/browser/TestUtils.test.ts

@@ -86,6 +86,9 @@ export class MockTerminal implements ITerminal {
   public attachCustomKeyEventHandler(customKeyEventHandler: (event: KeyboardEvent) => boolean): void {
     throw new Error('Method not implemented.');
   }
+  public attachCustomWheelEventHandler(customWheelEventHandler: (event: WheelEvent) => boolean): void {
+    throw new Error('Method not implemented.');
+  }
   public registerCsiHandler(id: IFunctionIdentifier, callback: (params: IParams) => boolean | Promise<boolean>): IDisposable {
     throw new Error('Method not implemented.');
   }