Make top-level matcher optional

[tadeuzagallo]

Right now, the grammar requires that you have a rule, and within a rule you can have multiple provider_matchers. A provider matcher is a combination of the provider to be used and a first matcher, e.g.

rule "example" {
  Swift::ClassDeclaration {
     // ...
  }
}

However, this doesn't allow matching multiple top-level siblings, e.g. in #9 where a rule is needed to match both a top-level protocol and a top-level class.

I think the most straight forward solution is to make the top level matcher optional, e.g.

rule "example" {
  Swift {
    var top_level_var := false

    ClassDeclaration {
      // ...
    }

    ProtocolDeclaration {
      // ...
    }
  }
}