This works surprisingly well. Whenever you type <ret> in Insert mode, and
the cursor is after the last prompt received from the REPL, everything after the
prompt up to (and including) the <ret> is sent. This works much like typing in a
terminal, except you can use the full power of Kakoune to edit your command-
line. You can also paste a whole block of text, and it won't be executed until
you press <ret>.

One downside is that undo does not work in a REPL buffer, even before the REPL
outputs something new.