Add prefers_default_focus and focus_scope modifiers

[carson-katri]

Closes #571
Closes #572

I wasn't able to see any effect on macOS/watchOS, and I'm unsure what that effect would be. Most examples are for tvOS, which is not currently supported by LVN.

I also included support for the resetFocus action, which can be triggered for a given namespace with push_event:

def handle_event("reset_focus", %{ "namespace" => namespace }, socket) do
  {:noreply, push_event(socket, :reset_focus, %{ namespace: namespace })}
end