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.

src/nvim/buffer_defs.h

@@ -996,6 +996,7 @@ typedef struct {
   bool noautocmd;
   bool fixed;
   bool hide;
+  int _cmdline_offset;
 } WinConfig;
 
 #define WIN_CONFIG_INIT ((WinConfig){ .height = 0, .width = 0, \
@@ -1009,7 +1010,8 @@ typedef struct {
                                       .style = kWinStyleUnused, \
                                       .noautocmd = false, \
                                       .hide = false, \
-                                      .fixed = false })
+                                      .fixed = false, \
+                                      ._cmdline_offset = INT_MAX })
 
 // Structure to store last cursor position and topline.  Used by check_lnums()
 // and reset_lnums().