Add support for Ctrl+h key

[cocoliliace]

From the conversation at https://lists.sr.ht/~rjarry/aerc-discuss/%3CD2PPJ3KYLYZ8.278RY369JS5RQ@liliace.dev%3E:

tmux is sending Ctrl+h as 0x08 (which is correct). Vaxis is
translating this to a backspace keypress - because some terminals use 0x08 as
backspace. Most use 0x7F as backspace nowadays.

I think this might be worthy of changing though. It's very uncommon to find a
terminal which uses 0x08 as backspace - and they almost universally allow
configuring this (tmux included).