better support for keyboard accessibility

[roeycohen]

In general using "Enter" on a toolbar that makes changes to lexical is problematic.

Lets say I have a button that toggles "bold" and it's triggered using the "Enter" key on the mouse-down event which calls dispatchCommand(FORMAT_TEXT_COMMAND...) - this causes lexical to get focus and then handle the Enter key which can replace the selected text with a new line.

adding the e.preventDefault to the event solves this problem but the focus still moves from the toolbar to the editor.

I'm not sure how to handle this...