Add support for enum `MIN_VALUE` and `MAX_VALUE` constants

Consider the following:
```groovy
enum Foo {
  BAR, BAZ
}
```

```java
class Pojo {
  void test() {
    Foo bar = Foo.MIN_VALUE;
  }
}
```

Groovy `enum` types provide public, static, final fields `MIN_VALUE` and `MAX_VALUE`.  These are not yet exposed via the Java model.