synIDattr(): true color awareness

In Vim, which doesn't true colors, synIDattr('Foo', 'fg') returns either
ctermfg or guifg depending on whether vim or gvim is running.

True colors naturally use GUI colors, so synIDattr() has to be adapted to
return guifg, if a TUI with enabled true colors is used.

src/nvim/eval.c

@@ -14699,6 +14699,8 @@ static void f_synIDattr(typval_T *argvars, typval_T *rettv)
     modec = TOLOWER_ASC(mode[0]);
     if (modec != 'c' && modec != 'g')
       modec = 0;        /* replace invalid with current */
+  } else if (ui_rgb_attached()) {
+    modec = 'g';
   } else {
     modec = 'c';
   }