Use inheritance for static interface

[mhfrantz]

Currently, there is no inheritance relationship between the generated interfaces representing the Java static API. It would be better to exploit that, in part to reduce the redundancy of inherited static API.

class Base {
    public static final String FOO = "foo";
}

class Derived extends Base {
    public static final String BAR = "bar";
}

This should result in something like this TS:

export module Base {
  export interface Static {
    FOO: string;
  }
}

export module Derived {
  export interface Static extends Base.Static {
    BAR: string;
  }
}