Add widget example

It would be nice to have an advanced example that shows how to make a widget that extends `_WidgetBase` and `_TemplatedMixin`. I see some examples in github repositories, but it would be good to get an example from SitePen that demonstrates best practices.

Start by reviewing existing examples in the typings repository and the referenced Dojo widget concepts, `_WidgetBase` and `_TemplatedMixin`. The work is done when the repository contains an advanced widget example from the SitePen project that demonstrates the requested best practices.

Written by the indexing model from the issue text.