Now there there is a new way to prompt for text, we can reimplement the
long abandoned editor_prompt function and use it to allow saving to new
files.

The new editor_prompt function takes a callback that will be called
after the user presses enter on the prompt.

Since this mode will also be used to accept input for setting the
filename, searching for a string and possibly more, it makes more sense
for this mode to be called prompt mode. There is also less ambiguity
between normal mode because sometimes that is also called command mode.

This way any extra stuff such as clearing the command line buffer or
setting the flag to ignore the extra first key can be automatically set.

This first step allows you to press the semicolon key to enter command
mode, where you can type into a command line. Currently, pressing enter
when in command mode will clear the command line but since there are no
commands implemented it doesn't matter what you type in there.

This is a bit of a silly experiment, but it already feel pretty
comfortable to have all text nagivation commands on the left hand
and manipulation commands on the right hand.

This name is a little bit better I think, and it will be nice to have a
distinction between this utility and the 'file' kind of buffer.