Lint rule to disable implicit dynamic values

[ikurek]

This should solve #84. The issue was raised a while ago, and the implicit dynamic option was already deprecated, but it was replaced with strict type checks. The changes here are mostly cosmetic, but overall they remove ~200 lint warnings in the codebase and remove implicit dynamic typing, which is discouraged in Dart for a while. The changes include:

• Enabled strict-casts and strict-inference lint rules, as a replacement for implicit-dynamic. Also, removed avoid_annotating_with_dynamic and avoid_types_on_closure_parameters since they conflict with strict-inference
• Added declared types to all methods that used implicit dynamic values
• Widened the scope of dynamic variables where it was possible. There were some cases where the variable/method did not need dynamic at all, as it's type was declared during asignement
• Removed unnecessary cast methods like transformState or transformStateChange. With declared variable types, there's no need to use methods that cast variables to their type in cases like this, a simple as cast is enough, and reduces a lot of boilerplate code
• Added void return types for Future methods that do not return any value. Previously these methods returned dynamic which was always a void type

[KacperKluka]

Looks good (after a skim-read) 👍