兼容较低的c#版本

RScriptContext.cs

@@ -362,7 +362,12 @@ namespace Convention.RScript
                 if (Variables.ContainsKey(varName))
                     Variables.SetValue(varName, varValue);
             }
-            return Variables.ToDictionary();
+            Dictionary<string, RScriptVariableEntry> result = new();
+            foreach (var (key, value) in Variables)
+            {
+                result[key] = value;
+            }
+            return result;
         }
     }
 }