Add "play" and "pause" events

[peaBerberian]

As I've been working a lot more with application code (media player UIs) lately, I saw that a real concept of whether playback was playing or paused was missing in the RxPlayer API.

You could approximate it by getting the player's state, but some states, like "BUFFERING" or "LOADING", even if they indicate that playback is not happening right now, do not indicate whether we're actually paused or not.

For example, once the buffering period is over, will we play directly or still be paused? Or, after "LOADING", will we play or just be paused on the first loaded frame?

One area where this concept is central to an application is for example to know whether the "play/pause" button should currently play or pause, which also dictates what the button should look like: if we're playing, this is a pause button which will pause playback, if we're paused, it will be a play button starting playback - and this, regardless of the current player's state.

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That's the reason the isPaused method (#1248) is planned for the future v3.31.0. While writing the beginning of that release's release note, I however noticed that the API wouldn't be complete without "play" and "pause" events, indicating respectively when the player exits a pause and when it enters it.

This PR adds those two events.

[peaBerberian]

Could also be written willAutoPlay === videoElement.paused but I though it was less understandable and that the double negation was, for once, more readable