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/Types.d.ts

@@ -32,6 +32,7 @@ export interface ITerminal extends InternalPassthroughApis, ICoreTerminal {
 }
 
 export type CustomKeyEventHandler = (event: KeyboardEvent) => boolean;
+export type CustomWheelEventHandler = (event: WheelEvent) => boolean;
 
 export type LineData = CharData[];