Add Positioned Widget to the Widget Catalog

Positioned Widget needs to be added to the Widget Catalog. Read `How to contribute to the Widget Catalog` in the [README](https://github.com/ACM-VIT/fludget/blob/master/README.md) for detailed steps. Make sure to include all the arguments(name, implementation, description, codeString, link and category). Refer to any widget in `widgetList.dart` and its own `*widget*.dart` file for better understanding. Showcase all possible parameters of the widget using different examples. (Refer to the official docs in case of any doubts)
Please attach screenshots of the same once done in the PR.