Use native beforeinput events to handle text insertion when possible

[rgrove]

In browsers that support it (currently only Safari has full support), the native beforeinput DOM event provides much more useful information about text insertion than React's synthetic onBeforeInput event.

By handling text insertion with the native event instead of the synthetic event when possible, we can fully support autocorrect, spellcheck replacements, and related functionality on iOS without resorting to hacks.

See the discussion in #1177 for more background on this change.

Note: Chrome has partial support for beforeinput, but doesn't allow the event to be canceled and doesn't reliably provide information about the affected range. Possibly because of this, Chrome doesn't report the existence of the beforeinput event, so it fails the feature test for that event and will still use the synthetic onBeforeInput code path.

I've tested this change extensively in macOS Safari, iOS Safari, Chrome, and Firefox. Don't have access to Edge at the moment, but I plan to test it next week (I expect it to fail the event feature test and use the synthetic onBeforeInput code path). More testing would be appreciated! I'm especially curious how this works on Android Chrome, but I don't have a device to test with right now.

Fixes #1176
Fixes #1177

[renchap]

@rgrove you can easily test for Edge on BrowserStack, Microsoft sponsored them to offer it for free: https://www.browserstack.com/test-on-microsoft-edge-browser#live-cloud

They also offer the service to OSS projects, it might be a good idea to contact them for this (I can do it if you want). Selenium testing might not be useful for now (but it would be great) but their live mode allows you to launch pretty much every browser/OS combinaison in 1 click which would help a lot to ensure Slate is compatible with the majors browsers.

PS: I am not affilited with them, but it saved me a lot of effort to achive compat with Edge on my app for free, so maybe it will for you too 🙂

[rgrove]

Fixed a lint error and resolved recently introduced conflicts with master. Also tested Edge and confirmed that it continues to use the synthetic onBeforeInput code path.

[ianstormtaylor]

Thank you @rgrove this is a really great PR, and awesome research/testing. Thank you very much! Hopefully more browsers are quick to implement this and it will begin solving more mobile issues.