patch 8.2.1853: "to_f" is recognized at "topleft" modifier

Problem:    "to_f" is recognized at "topleft" modifier.
Solution:   Do not recognize modifer when "_" follows. (closes #7019)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index cf06b5c..478a1e5 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -454,7 +454,6 @@
 enddef
 
 def Test_assignment_default()
-
   # Test default values.
   var thebool: bool
   assert_equal(v:false, thebool)
@@ -571,6 +570,10 @@
     assert_equal(43, w)
     var t: number = 44
     assert_equal(44, t)
+
+    var to_var = 0
+    to_var = 3
+    assert_equal(3, to_var)
   END
   CheckScriptSuccess(lines)