feat: relax rule to polyvar_declaration

[aspeddro]

Here an example.

type subtype<'w, 'ty> = Stream.subtype<[> kind<'w, 'r>] as 'ty>

(ERROR [0, 0] - [1, 0]
  (type_identifier [0, 5] - [0, 12])
  (type_parameters [0, 12] - [0, 21]
    (type_identifier [0, 13] - [0, 15])
    (type_identifier [0, 17] - [0, 20]))
  (generic_type [0, 24] - [0, 54]
    (type_identifier_path [0, 24] - [0, 38]
      (module_identifier [0, 24] - [0, 30])
      (type_identifier [0, 31] - [0, 38]))
    (ERROR [0, 38] - [0, 46]
      (polyvar_declaration [0, 42] - [0, 46]
        (type_identifier [0, 42] - [0, 46])))
    (type_arguments [0, 46] - [0, 54]
      (type_identifier [0, 47] - [0, 49])
      (type_identifier [0, 51] - [0, 53])))
  (ERROR [0, 60] - [0, 62]))

[nkrkv]

Thank you! Marged.