"no-cross-overload" should not fire on static and regular operations with same identifier

[tidoust]

Web IDL was recently updated to allow static operations to have the same name as regular operations, see whatwg/webidl#1097

The checker seems happy with that as long as these operations are defined in the same base definition. However, the checker reports a no-cross-overload error when the definitions are scattered across partials or mixins. For instance, the checker reports the error for the following IDL:

[Exposed=*]
interface Test {
  undefined operation();
};

partial interface Test {
  static undefined operation();
};

Since operations are not of the same kind (regular vs. static), they should not be considered as overloaded operations, and I think the preceding IDL is valid.

Real world example is the recent update to fetch to add a static Response.json operation: whatwg/fetch@b3bfd0c. In that case, the static operation is on the base mixin interface Body and the regular operation on Response. The Web IDL checker complains with the same "no-cross-overload" error.