patch 9.0.1123: class function not implemented yet

Problem:    Class function not implemented yet.
Solution:   Implement defining and calling a class function.
diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index 964f78e..9dd6f83 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -377,7 +377,7 @@
          static _secret = 7
          public static  anybody = 42
 
-         def AddToCounter(nr: number)
+         static def AddToCounter(nr: number)
            counter += nr
          enddef
       endclass
@@ -403,6 +403,32 @@
   v9.CheckScriptSuccess(lines)
 enddef
 
+def Test_class_function()
+  var lines =<< trim END
+      vim9script
+      class Value
+        this.value = 0
+        static objects = 0
+
+        def new(v: number)
+          this.value = v
+          ++objects
+        enddef
+
+        static def GetCount(): number
+          return objects
+        enddef
+      endclass
+
+      assert_equal(0, Value.GetCount())
+      var v1 = Value.new(2)
+      assert_equal(1, Value.GetCount())
+      var v2 = Value.new(7)
+      assert_equal(2, Value.GetCount())
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 def Test_class_object_to_string()
   var lines =<< trim END
       vim9script