[@mantine/core] PinInput: shift focus to prev element on backspace

[mrbnclt]

This fixes #4432

Scenarios:

• If the focus is on the last element and there is a value, hitting the backspace will remove the value and stay focused on that element.
• If the focus is on the last element and there is no value, hitting the backspace will shift the focus to the previous element.
• If the focus is on any input element except the last element, hitting the backspace will remove the value and shift the focus to the previous element.

pininput-focus-shift-fix.mp4

[rtivital]

Thanks!