feat(lastValue): Adds lastValue method to Observable

[benlesh]

PROPOSED CHANGE

This is meant to be a more well-behaved, a better-named, replacement for toPromise.

Resolves #5075

[ncjamieson]

@benlesh Could this be refactored into the static method that was discussed in the core team meeting?

import { lastValue, of } from "rxjs";

function example () {
  const value = await lastValue(of(42));
}

IIRC, I might have previously had reservations about the name. If that was the case, I've changed my mind. I think lastValue is nice and is a big improvement on toPromise. (The name describes the behaviour and the types will tell people that it returns a promise.)

[cartant]

Closing this in favour of #5295