If backspace pressed in empty box, move to previous box

[avraham-mayer]

i noticed that if backspace is pressed in a box with a character in it, the character is erased and focus is transferred to the previous box
during editing when a character is entered focus is transferred to the next box - which is empty
if backspace is pressed in the empty box, nothing happens because the onChange function is not triggered by a backspace in an empty text field
i added a check in the onKey of the existing RawKeyboardListener to check if backspace is pressed in an empty box, and move to the previous box

[awaik]

Hi!
Thank you for your contribution - merged and published
https://pub.dev/packages/flutter_verification_code/changelog