refactor: rename function prefix mb to the more accurate utf_cp (#19590)

The "cp" stands for codepoint.

Closes https://github.com/neovim/neovim/issues/7401

src/nvim/ops.c

@@ -1828,7 +1828,7 @@ static void mb_adjust_opend(oparg_T *oap)
 {
   if (oap->inclusive) {
     char *p = (char *)ml_get(oap->end.lnum);
-    oap->end.col += mb_tail_off(p, p + oap->end.col);
+    oap->end.col += utf_cp_tail_off(p, p + oap->end.col);
   }
 }