Add rule for "const": generate Java constants

[volomas]

As of now, generator doesn't take into account "const" keyword.
The value of "const" is simply lost during generation.

In this PR, I've added ConstRule that generates java constant:
public static final <Type> <snake_case_var_name> = <value>.

This allows to use this value in code like this:

var myClass = new GeneratedClass();
myClass.setMyProperty(GeneratedClass.MY_PROPERTY);

... or just when I need to refer to that constant somewhere in the code.

It doesn't set the generated field value to "const" value, because it would mimic "default", so I wasn't sure if that's a good idea.