feat(ui): use builtin completion popupmenu with ext_cmdline (#31269)

Problem:  UIs implementing ext_cmdline/message must also implement
          ext_popupmenu in order to get cmdline completion with
          wildoptions+=pum.
Solution: Allow marking a window as the ext_cmdline window through
          nvim_open_win(), including prompt offset. Anchor the cmdline-
          completion popupmenu to this window.

runtime/lua/vim/_meta/api_keysets.lua

@@ -312,6 +312,7 @@ error('Cannot require a meta file')
 --- @field noautocmd? boolean
 --- @field fixed? boolean
 --- @field hide? boolean
+--- @field _cmdline_offset? integer
 
 --- @class vim.api.keyset.win_text_height
 --- @field start_row? integer