Make `Term::prompt_multiline` support Alt+Backspace

[avighnac]

On a normal Linux command line, Alt+Backspace erases an entire word. Supporting that would be a nice enhancement.

[staxfur]

Yeah, we are currently working on improving the base functional and overall design. I sadly hadn't time yet to look into the prompt code. But I'll look into this when we are getting there, thanks!