patch 9.0.0350: :echowindow does not work in a compiled function

Problem:    :echowindow does not work in a compiled function.
Solution:   Handle the expression at compile time.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 5aec8ff..bdefc87 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -2011,6 +2011,13 @@
   # output goes anywhere
 enddef
 
+def Test_echowindow_cmd()
+  var local = 'local'
+  echowindow 'something' local # comment
+  # output goes in message window
+  popup_clear()
+enddef
+
 def Test_for_outside_of_function()
   var lines =<< trim END
     vim9script