fix: support formSheet modal and floating keyboard

[toshi0383]

fixes: #33
fixes: #37

It turns out that formSheet automatically adjusts its position when keyboard is shown.
So I needed to fallback to use keyboardDidChangeFrameNotification instead of keyboardWillChangeFrameNotification in this use case.
This causes animation delay, but otherwise I couldn't calculate correct keyboardHeight inside formSheet.

kblayout-formsheet.mp4

Usage is simple enough.
Just set Keyboard.shared.presentedViewController when you know what you're doing.

Keyboard.shared.presentedViewController = self
vStack.bottomAnchor.constraint(lessThanOrEqualTo: view.keyboardLayoutGuideNoSafeArea.topAnchor).isActive = true