Analogous to nodejs's `on('data', …)` interface, here on_key is the "add
listener" interface.
ref 3ccdbc570d#12536
BREAKING_CHANGE: vim.register_keystroke_callback() is now an error.
BSD Make will give preference to a BSDmakefile in the same directory
over a generic Makefile; this can be used to instruct BSD users to build
neovim with GNU Make (gmake) instead.
Otherwise, a flood of syntax errors stemming from the GNU-specific
Makefile will be displayed - which most BSD users are accustomed to, but
may confuse beginners nevertheless.