Add location to `UnknownFeatureError`

[nobodywasishere]

Can probably be solved via:

def [](lookup, location = nil) : T
  # ...

  if feature.nil?
    exception = UnknownFeatureError.new(self.name, lookup)

    if location
      exception.at(location)
    end

    raise exception
  end

  feature
end

private def parse_tag
  # ...
  tag = @env.tags[name_token.value, current_token]
  # ...
end