new app template for eXist-db apps

[JoernT]

As roaster is pretty generic and superior to the old controller.xql architecture there should be an update to the app template that e.g. eXide is generating for new apps.

It should contain the necessary barebones of api.json file(s) and matching controller.xql (as long as that still exists).

The feature is probably not part of this project but needs to kicked off somewhere ;)

[line-o]

https://github.com/eXist-db/generator-exist would be a good fit :)

[duncdrum]

yup PRs with a new template are always welcome.