feat(ui): don't show unfocusable windows in :tabs, 'tabline' #27984

Problem:  Floating windows with focusable set to false can reasonably be
          expected to be UI elements but are listed in some outputs that
          should contain only regular windows.
Solution: Hide unfocusable floating windows from the default tabline and
          :tabs.

src/nvim/ex_docmd.c

@@ -5507,6 +5507,8 @@ static void ex_tabs(exarg_T *eap)
     FOR_ALL_WINDOWS_IN_TAB(wp, tp) {
       if (got_int) {
         break;
+      } else if (!wp->w_config.focusable) {
+        continue;
       }
 
       msg_putchar('\n');