fix: fromEvent() type with resultSelector

[rocwang]

Description:

This PR simply fixes the return type of one overloaded form of fromEvent() that accepts a resultSelector.

With a resultSelector: (event: T) => R, fromEvent<T, R> should return a Observable<R>, instead of Observable<T>.

Related issue (if exists): N/A

[cartant]

This change affects the public API, so the api_guardian files will need to be updated by running npm run api_guardian:update.

[rocwang]

This change affects the public API, so the api_guardian files will need to be updated by running npm run api_guardian:update.

@cartant Thanks for the reminder! The api_guardian files are updated now.

[cartant]

I've had a closer look at the dtslint tests to see why this wasn't picked up. We should add two tests similar to these and should include the options argument so that those signatures are tested:

https://github.com/ReactiveX/rxjs/blob/7.1.0/spec-dtslint/observables/fromEvent-spec.ts#L23-L30

FYI, the dtslint tests are run using npm run dtslint.

[rocwang]

Good call. I've added 4 more dtslint tests there for fromEvent. Now all overload forms should be covered.

[cartant]

LGTM. Thanks.

[rocwang]

Just fixed a typo.