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/mbyte.c

@@ -1532,34 +1532,6 @@ int mb_cptr2char_adv(char_u **pp)
   return c;
 }
 
-/*
- * Check whether we are dealing with Arabic combining characters.
- * Note: these are NOT really composing characters!
- */
-int arabic_combine(
-    int one,                    /* first character */
-    int two                     /* character just after "one" */
-    )
-{
-  if (one == a_LAM)
-    return arabic_maycombine(two);
-  return FALSE;
-}
-
-/*
- * Check whether we are dealing with a character that could be regarded as an
- * Arabic combining character, need to check the character before this.
- */
-int arabic_maycombine(int two)
-{
-  if (p_arshape && !p_tbidi)
-    return two == a_ALEF_MADDA
-      || two == a_ALEF_HAMZA_ABOVE
-      || two == a_ALEF_HAMZA_BELOW
-      || two == a_ALEF;
-  return FALSE;
-}
-
 /*
  * Check if the character pointed to by "p2" is a composing character when it
  * comes after "p1".  For Arabic sometimes "ab" is replaced with "c", which