Add `button` number property to mouse events

[appden]

Proposal: Add button number property to mouse events

Summary

React Native macOS provides the button property (0 for left, 2 for right) here, which comes from the DOM MouseEvent.button API.

On React Native Windows, the mouse events have "non-standard" boolean properties (e.g. isLeftButton, isRightButton, etc.) set here. This proposal is to add button property and hopefully deprecate the others so that code built for both macOS and Windows doesn't need separate checks to detect right clicks for each platform.

[NickGerleman]

Would be interested in your feedback on a WIP document that @kikisaints is working on related to mouse events. There's actually a comment open right now asking about what we want mouse events to look like.

[NickGerleman]

There's kind of a separate question of how we transition from one set of events/event props to another. Untyped usage makes deprecation particularly hairy. I think one of the ideas was a set of new pointer event handlers where we started with a min-set of supported properties, so we can build up a new event without breaking the old.

[chrisglein]

Assigning to @kikisaints to reconcile with her in-flight doc of mouse API changes.