Typed fromEvent for web

[waterplea]

When we use fromEvent in web we can extract type of event from the map of event names that are present in TypeScript.

It is also often helpful to have currentTarget of that event to have the same type as the target we listen this event on so we do not have to typecast low level EventTarget when we work with it down the RxJS line.

currentTarget type is questionable because it changes. currentTarget only matches the element we listen event on directly in the callback. So this might not be a good idea but it's up for a debate :)

Here's a little proof of concept with events on Element, Window and Document with fallback to current fromEvent behavior where we just have generic:

https://stackblitz.com/edit/angular-typed-fromevent-delux

This could be a starting point for discussion/implementation.