patch 8.2.1271: Vim9: Error for Funcref function argument type

Problem:    Vim9: Error for Funcref function argument type.
Solution:   Find the actual function type if possible. (issue #6507)

src/testdir/test_vim9_func.vim

@@ -270,6 +270,19 @@ def Test_call_funcref()
     assert_equal(123, Funcref())
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    def RetNumber(): number
+      return 123
+    enddef
+    def Bar(F: func: number): number
+      return F()
+    enddef
+    let Funcref = function('RetNumber')
+    assert_equal(123, Bar(Funcref))
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 let SomeFunc = function('len')