Relax codegen mixin rules

[katis]

Allows you to write codegen like this (the class Admin = AdminBase with _$Admin syntax still works):

class Admin extends AdminBase with _$Admin {
  Admin(int id) : super(id);
}

abstract class AdminBase extends User implements Store {
  AdminBase(int id) : super(id);

  @observable
  String userName = 'admin';

  @observable
  ObservableList<String> accessRights = ObservableList();
}

Makes you reimplement base class constructor signature, but should work better with json_serializable

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   96.55%   96.58%   +0.02%     
==========================================
  Files          29       29              
  Lines         784      790       +6     
==========================================
+ Hits          757      763       +6     
  Misses         27       27

Flag	Coverage Δ
#example	100% <100%> (ø)	⬆️
#flutter_mobx	100% <ø> (ø)	⬆️
#mobx	96.31% <80%> (-0.01%)	⬇️
#mobx_codegen	97.74% <80%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
example/lib/src/generator_example.g.dart	100% <100%> (ø)	⬆️
example/lib/src/generator_example.dart	100% <100%> (ø)	⬆️
mobx_codegen/lib/src/mobx_codegen_base.dart	95.16% <80%> (-0.08%)	⬇️

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