refactor(options): remove set_string_option_direct()

Problem: `set_string_option_direct()` contains a separate codepath specifically for setting string options. Not only is that unnecessary code duplication, but it's also limited to only string options.

Solution: Replace `set_string_option_direct()` with `set_option_direct()` which calls `set_option()` under the hood. This reduces code duplication and allows directly setting an option of any type.

src/nvim/diff.c

@@ -1438,7 +1438,8 @@ void diff_win_options(win_T *wp, bool addbuf)
     }
     wp->w_p_fdm_save = xstrdup(wp->w_p_fdm);
   }
-  set_string_option_direct_in_win(wp, kOptFoldmethod, "diff", OPT_LOCAL, 0);
+  set_option_direct_for(kOptFoldmethod, STATIC_CSTR_AS_OPTVAL("diff"), OPT_LOCAL, 0, kOptReqWin,
+                        wp);
 
   if (!wp->w_p_diff) {
     wp->w_p_fen_save = wp->w_p_fen;