Previously we wouldn't handle Backspace in dead key state correctly:
instead of removing what was typed, we'd insert the text that was in the
dead key state.

Example: on a US English layout, press `opt-u` to end up in a dead key
state with `¨` waiting for the next character to be typed. Type
`backspace`. The `¨` should be removed, but it's not.

With this change, the `backspace` is interpreted instead of being
ignored.

Release Notes:

- Fixed backspace not working for dead keys (i.e. when typing accents or
umlauts)

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>