patch 8.2.1352: Vim9: no error for shadowing a script-local function

Problem:    Vim9: no error for shadowing a script-local function by a nested
            function.
Solution:   Check for script-local function. (closes #6586)

src/testdir/test_vim9_func.vim

@@ -174,6 +174,20 @@ def Test_nested_global_function()
       Outer()
   END
   CheckScriptFailure(lines, "E122:")
+
+  lines =<< trim END
+      vim9script
+      def Func()
+        echo 'script'
+      enddef
+      def Outer()
+        def Func()
+          echo 'inner'
+        enddef
+      enddef
+      defcompile
+  END
+  CheckScriptFailure(lines, "E1073:")
 enddef
 
 def Test_global_local_function()