gql lexer cannot parse language name along with script

[windoze]

I.e. this will not return any result

mutation {
  set {
    <1> <text> "This is a colour"@en-us .
  }
}

{
  q(func: uid(1)) {
    text@en-us
  }
}

But this will return result correctly

mutation {
  set {
    <1> <text> "This is a colour"@en .
  }
}
{
  q(func: uid(1)) {
    text@en
  }
}

Looks like gql parser has some issues to parse language tag with '-' in the middle.

[windoze]

I've add a hack in #1428, which allows queries to use underscores, i.e. "en_us", while mutations and RDF are still using "-".