UI: always use contrete colors for default_colors_set

But add an escape hatch needed for external TUI, so it still can use
terminal emulator defaults.

src/nvim/ui.h

@@ -18,6 +18,7 @@ typedef enum {
   kUILinegrid,
   kUIMultigrid,
   kUIHlState,
+  kUITermColors,
   kUIExtCount,
 } UIExtension;
 
@@ -29,6 +30,7 @@ EXTERN const char *ui_ext_names[] INIT(= {
   "ext_linegrid",
   "ext_multigrid",
   "ext_hlstate",
+  "ext_termcolors",
 });