feat(options): add 'winpinned' to pin a window #39157

Problem:
- Unable to "pin" a window to prevent closing without specifically
  being targeted.
- :fclose closes hidden windows (even before visible windows).

Solution:
- Add 'winpinned' window-local option. When set, window is skipped by
  :fclose and :only. Pin the ui2 cmdline window (which should always be
  visible), so that it is not closed by :only/fclose.
- Skip over hidden (and pinned) windows with :fclose.

Co-authored-by: glepnir <glephunter@gmail.com>

test/functional/ui/messages2_spec.lua

@@ -432,7 +432,7 @@ describe('messages2', function()
     screen:expect([[
       ^                                                     |
       {1:~                                                    }|*12
-                                                           |
+      foo [+1]                                             |
     ]])
   end)