Expose auto_suggest.resume_hinting, fix resume on backspace

[krassowski]

This is a pre-requisite of #13992 but the shortcut is disabled by default by never filter. The idea here is that this could be merged as-is (ideally after rebasing on top of #13991) to allow user testing. Once this is in, users can emulate part of the behaviour proposed in #13992 with the following snippet:

custom_shortcuts = [
    {
        "command": "IPython:auto_suggest.resume_hinting",
        "new_keys": ["right"],
        "new_filter": "default_buffer_focused & ((vi_insert_mode & ebivim) | emacs_insert_mode)"
    }
]
%config TerminalInteractiveShell.shortcuts = custom_shortcuts