refactor: facelift both Envied and EnviedGenerator to Dart 3.0

[techouse]

• bump the version to 0.4.0
• raise Dart SDK minimum to 3.0.0
• update dependencies (except analyzer)
• add a const constructor to the EnviedGenerator
• add the final Dart 3 class modifier Envied, EnviedField, EnviedGenerator and Parser
• declare all the Parser methods as static
• refactor the generator by:
  • adding explicit types
  • explicitly declaring nested functions
  • converting all immutable var variables to final
• generate all _Env classes with the final Dart 3 class modifier, i.e.

  final class _Env16 {
    static const String testDefaultParam = 'test_';
  }

• update example

[Ax0elz]

Can this be pushed through? This is much needed as generation currently fails on the latest version of Flutter/Dart 3.0

[techouse]

generation currently fails on the latest version of Flutter/Dart 3.0

Err, works fine for me in Dart 3.0 and 3.1 without this PR. You probably have some user error that's preventing you from successfully generating envs.

Try to clone the examples from this repo and test the generation of those.