Optionally use native 'beforeinput' events instead of React's synthetic event

[ShannonLCapper]

React currently does not support using the native beforeinput event supported by some browsers and instead polyfills the event using either textInput or keypress events (more context here). beforeinput events with Level 2 Input support (meaning that they fire and are cancellable) currently only exists in Safari (more context here)

However, we have a bug that specifically repros in the version of Safari that runs in Office Add-ins for Mac--calling preventDefault() on a textInput event will cause the system OS alert noise.

This PR optionally bypasses React's synthetic onBeforeInput event (which uses textInput events under the covers in Safari) and allows consumers to specify that we should instead use the native beforeinput event directly if they're supported in the browser. This is the same thing Slate does, and allows us to get around our very specific error case in Outlook for Mac. However, since this opens us up to opportunities for unexpected behavior, this is opt-in only to reduce our risk surface-area.

[max-winderbaum]

LGTM