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/ex_getln.c

@@ -3008,10 +3008,9 @@ static int cmd_startcol(void)
 int cmd_screencol(int bytepos)
 {
   int m;  // maximum column
-
   int col = cmd_startcol();
   if (KeyTyped) {
-    m = Columns * Rows;
+    m = cmdline_win ? cmdline_win->w_view_width * cmdline_win->w_view_height : Columns * Rows;
     if (m < 0) {        // overflow, Columns or Rows at weird value
       m = MAXCOL;
     }