Instead of having all sendstring keycode mappings in the main quantum.c
file, give each one its own file in keymap_extras that can be #included
in a user's keymap. If one is included, it will define the appropriate
lookup tables and overwrite the weak definitions in quantum.c.
(Including more than one sendstring definition will fail at compile
time.)
Update @rai-suta's test keymap to match, as well as the documentation.
Refactor new-ish JIS_KEYCODE send_string implementation with existing
send_string
Reshuffle JIS in line with other alternative keycodes for sendstring,
and make them all accessible via compile-time options
Add a separate function to allow sending a string with a delay.
