fix(ui2): respect options set during startup after enabling ui2 #41591

Problem: options related to ui2 (like `fillchars` with `msgsep`) do not
  take effect if set during startup after enabling ui2.

Solution: explicitly check just after startup if relevant options were
  changed during startup.

runtime/lua/vim/_core/ui2.lua

@@ -243,6 +243,7 @@ function M.enable(opts)
   if vim.v.vim_did_enter == 0 then
     vim.schedule(function()
       check_cmdheight(vim.o.cmdheight)
+      M.msg.on_option_changed()
     end)
   end