Pull out sendstring variations to their own files.

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.