There is no TS type for vaadin-contextmenu event

[Haprog]

As a developer when using TypeScript I want to be able to easily discover which event type I should use for an event handler when binding to @vaadin-contextmenu. There should probably be a new event type for it and it should specify the type of the detail object too.

See:

• web-components/packages/vaadin-context-menu/src/vaadin-contextmenu-event.js

  Lines 95 to 96 in b682812

  	const ev = new Event('vaadin-contextmenu', { bubbles: true, cancelable: true, composed: true });
  	ev.detail = { x, y, sourceEvent };

Would also be great if you could specify the type of event.currentTarget via generics to make this kind of usage simpler (if possible) so that the user would not need to do so much casting (with as):

Related: #1015