textinput/textarea: alias shift+backspace to backspace

[jabagawee]

On keyboard protocols that distinguish modifiers (e.g. the kitty keyboard protocol), shift+backspace arrives as a separate key from backspace, so users can't delete a character backward while holding shift in those environments. Most other editors treat the two identically.

Proposing to add shift+backspace alongside backspace and ctrl+h on the DeleteCharacterBackward binding in both textinput and textarea. I have this implemented and tested on a fork.