Add support for initializing member variables

The implementation follow TS rules:
- when the memberVariables option is enabled, the constructor must call the super constructor,
- when the memberVariables option is enabled and a class has initialized instance variables, the call to the super constructor must be the first statement in the class own constructor,
- instance variables initializers are evaluated when the class is instantiated.
The code currently leaves in the ClassTransformer. It would be a mess to add it to the parser (initialized instance vars can come after the constructor).