Introduce GA_APPEND()

This macro is used to append an element to a growable array. It replaces this
common idiom:

   ga_grow(&ga, 1);
   ((item_type *)ga.ga_data)[ga.ga_len] = item;
   ++ga.ga_len;

src/nvim/spell.c

@@ -9138,8 +9138,7 @@ someerror:
           if (c < 0) {
             goto someerror;
           }
-          ga_grow(&ga, 1);
-          ((char_u *)ga.ga_data)[ga.ga_len++] = c;
+          GA_APPEND(char_u, &ga, c);
           if (c == NUL)
             break;
         }