patch 8.2.2188: Vim9: crash when calling global function from :def function

Problem:    Vim9: crash when calling global function from :def function.
Solution:   Set the outer context.  Define the partial for the context on the
            original function. Use a refcount to keep track of which ufunc is
            using a dfunc. (closes #7525)

src/testdir/test_vim9_func.vim

@@ -299,6 +299,7 @@ def Test_nested_global_function()
       Outer()
   END
   CheckScriptFailure(lines, "E122:")
+  delfunc g:Inner
 
   lines =<< trim END
       vim9script
@@ -1565,6 +1566,25 @@ def Test_global_closure()
   bwipe!
 enddef
 
+def Test_global_closure_called_directly()
+  var lines =<< trim END
+      vim9script
+      def Outer()
+        var x = 1
+        def g:Inner()
+          var y = x
+          x += 1
+          assert_equal(1, y)
+        enddef
+        g:Inner()
+        assert_equal(2, x)
+      enddef
+      Outer()
+  END
+  CheckScriptSuccess(lines)
+  delfunc g:Inner
+enddef
+
 def Test_failure_in_called_function()
   # this was using the frame index as the return value
   var lines =<< trim END