fix(extui): cmdline visibility, cmdheight cursor position (#33774)

Problem:  The cmdline popupmenu is hidden behind extui windows.
          'showmode' message is drawn over the cmdline.
          Changing the 'cmdheight' to accommodate space for the text in
          the cmdline may change the current cursor position.
Solution: Ensure kZIndexMessages < zindex < kZIndexCmdlinePopupMenu.
          Clear the 'showmode' message when the cmdline level is negative.
          Temporarily set 'splitkeep' = "screen" when changing the 'cmdheight'.

runtime/lua/vim/_extui/messages.lua

@@ -331,7 +331,7 @@ function M.msg_clear() end
 ---
 ---@param content MsgContent
 function M.msg_showmode(content)
-  M.virt.last[M.virt.idx.mode] = content
+  M.virt.last[M.virt.idx.mode] = ext.cmd.level < 0 and content or {}
   M.virt.last[M.virt.idx.search] = {}
   set_virttext('last')
 end
@@ -386,7 +386,6 @@ function M.set_pos(type)
       height = height,
       row = win == ext.wins[ext.tab].box and 0 or 1,
       col = 10000,
-      zindex = type == 'more' and 299 or nil,
     })
     if type == 'box' then
       -- Ensure last line is visible and first line is at top of window.