WACC Examples Selector

The file upload box could be expanded with the ability to pick directly from the most up-to-date wacc_examples. This can be done by holding a copy of these files server-side so they can be served up by the backend.

On the frontend side, however, there is no way to get a file-picker that will see the backend, so we will need our own dir-tree element to be able to display the choices to the user.

On the CLI side, we'll need some kind of flag to be able to pick the example.