Pull out sendstring variations to their own files.

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.

keyboards/planck/keymaps/rai-suta/keymap.c

@@ -3,6 +3,8 @@
 #include "planck.h"
 #include "version.h"
 
+#include "sendstring_jis.h"
+
 // Keycode defines
 #define _______ KC_TRNS
 #define XXXXXXX KC_NO