Ignore #: annotation on attr_* calls

[soutaro]

This improves the compatibility with rbs-inline, where attr_* calls may be annotated for the type of the attributes.

Steep now ignores #: annotations given to attr_reader, attr_accessor, and attr_writer calls without receivers. If you really need some type annotation to your custom attr_reader (likes) calls, you need to add self. receiver or surround them with parens.

class Foo
  # Type declaration for rbs-inline, Steep ignores the annotation
  attr_reader :foo #: String

  # Steep detects the annotation because it's with `self` receiver
  self.attr_reader :foo #: String

  # Steep detects the annotation because of the parens
  (attr_reader :foo) #: String
end

[soutaro]

rbs-inline currently uses #:: syntax, but I will make it to #:. And this PR is needed to avoid the conflict.