Fix auto completion and code navigation for script containing class that extends `Script`

Consider the following:
```groovy
abstract class Something extends Script {
  protected Something(Binding binding) {
    super(binding)
  }
}
//@groovy.transform.BaseScript Something something
def foo = "bar baz"
println "whatever"
```

The script body code is highlighted properly, but code hover/navigation and auto-completion suggestions are not available.