Implement vi_kill_line_prev

[jez]

This operation is mentioned and bound to ^U in both vi_command.rb
and vi_insert.rb, but there is no definition of it.

Both Vi and Emacs use the same keystroke to do the same behavior, so
I've chosen to use alias_method to make the implementation small,
rather than duplicating the method and re-implementing it.

[jez]

This is the behavior I see before making this change:

But after I make this change, I see that ^U works.

If you'd like me to write a proper test for this, I can look into how to do that, but I would love suggestions on where to put it.

[aycabta]

Thank you so much! CI failed, but this is a problem that has been resolved in master.