T352881: Add Bold & Italics shortcuts to Native Editor

[christianbilodeau]

Phabricator: T352881

Notes

• Tested only on Simulator with my Mac's keyboard

Test Steps on Simulator

1. Go to editor for any page
2. Make sure hardware keyboard is connected (I/O -> Keyboard -> Connect hardware keyboard)
3. Select some text
4. Press CMD+B or CMD+I