增加对Decimal类型的支持

Helper/ModelTypeGenerator.cs

@@ -34,11 +34,11 @@ namespace FlutterCodeGenerator.Helper
                 {
                     return new IntNullableModelType(argName);
                 }
-                else if (type == typeof(float) || type == typeof(double))
+                else if (type == typeof(float) || type == typeof(double) || type == typeof(decimal))
                 {
                     return new DoubleModelType(argName);
                 }
-                else if (type == typeof(float?) || type == typeof(double?))
+                else if (type == typeof(float?) || type == typeof(double?) || type == typeof(decimal?))
                 {
                     return new DoubleNullableModelType(argName);
                 }