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>

runtime/lua/vim/_core/ui2.lua

@@ -130,6 +130,8 @@ function M.check_targets()
           hl = 'Normal:MsgArea'
         elseif type == 'msg' then
           hl = search_hide
+        elseif type == 'cmd' then
+          api.nvim_set_option_value('winpinned', true, { scope = 'local' })
         end
         api.nvim_set_option_value('winhighlight', hl, { scope = 'local' })
       end)