Move arabic.h #defines from arabic.h to arabic.c

- 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)

src/globals.h

@@ -1136,9 +1136,4 @@ EXTERN char *ignoredp;
  */
 # include "farsi.h"
 
-/*
- * Optional Arabic support. Include it here, so EXTERN and INIT are defined.
- */
-# include "arabic.h"
-
 #endif /* NEOVIM_GLOBALS_H */