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

test/playwright/TestUtils.ts

@@ -75,6 +75,7 @@ type TerminalProxyCustomOverrides = 'buffer' | (
   'options' |
   'open' |
   'attachCustomKeyEventHandler' |
+  'attachCustomWheelEventHandler' |
   'registerLinkProvider' |
   'registerCharacterJoiner' |
   'deregisterCharacterJoiner' |