[RFC] Move arabic.h #defines from arabic.h to arabic.c and remove include of arabic.c

     - Turn ARABIC_CHAR into the arabic_char function
     - Move arabic_shape() decl from main.h to arabic.h
     - Move arabic_combine() and arabic_maycombine() from mbyte.c to
       arabic.c as these functions use the #defines I moved.
     - Remove the unnecessary include of arabic.h in globals.h
     - Remove include of arabic.c (sic) in main.c (change CMakeLists.txt to compile
       arabic.c normally)

Test plan: open a `Arabic-Lorem-Ipsum.txt` file :)