feat: add nnbd support and require dart 2.12.0 (jeroen-meijer#15)

* feat: add nnbd support and require dart 2.12.0

* fix ci dart version, remove old meta import, fix equatable props issue

* set version to 2.0.0

* allow more than one version for equatable

* make conversiontoken value nnbd

fix: upgrade equatable to fix dartdoc warnings (jeroen-meijer#14)

docs: add library docs (jeroen-meijer#13)

- add library docs
- move test depedency to dev_dependencies
- MOAR BADGES <3

ci: fix typo in publish step (jeroen-meijer#12)

we all make mistakes ¯\_(ツ)_/¯

ci: fix release draft format