Java 9 support

[JonathanxD]

One of the features of CodeAPI 4 is Java 9 Support, only Module structure was implemented, now that Java 9 was released, some reviews is needed to achieve the full support, example, TryWithResources only accepts VariableDeclaration, in Java 9, you can use an existing variable in try-with-resources.

Also, CodeAPI should be modularized, this will happen in the future (in some minor release), and later, in 5.0.0, all CodeAPI code base will be compiled targeting Java 9.

Note: _ is no longer a valid identifier, but as CodeAPI does not care about valid identifier, this check will be added to JavaValidator.