create a Lumen demo app

https://github.com/Astrotomic/stancy-lumen-demo

The demo app should show the basic features and in best case contain some integration tests using [spatie/phpunit-snapshot-assertions](https://github.com/spatie/phpunit-snapshot-assertions). Only simple tests like: if I call this url/route I expect this response/html/snapshot.

relates to #9

Start with the linked Astrotomic/stancy-lumen-demo repository and inspect issue #9 for related scope. Review Stancy’s basic features and the proposed spatie/phpunit-snapshot-assertions integration tests; the work is done when the demo app shows those features and simple URL or route response and snapshot tests pass.

Written by the indexing model from the issue text.