Add support for module level attributes (#58)

[andryak]

Summary

This is valid Hack code that declares a module-level attribute:

<?hh

<<file: SomeClass(SomeOtherClass::class)>>

Yet, the parser does not recognize the syntax:

(script
      (ERROR
        (attribute_modifier
          (ERROR
            (identifier))
          (qualified_identifier
            (identifier))
          (arguments
            (argument
              (scoped_identifier
                (qualified_identifier
                  (identifier))
                (identifier)))))))

This PR adds support for this syntax.

Requirements

• [x] I've added tests for any new code and ran npm run test-corpus to make sure all tests pass.
• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

[salesforce-cla[bot]]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Andrea Aquino a***@f***.com. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce Inc. Contributor License Agreement and this Pull Request will be revalidated.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @andryak to sign the Salesforce Inc. Contributor License Agreement.