feat: add broadcast_shapes to the specification

[kgryte]

This PR

• resolves RFC: add broadcast_shapes to the specification #893 by adding support for broadcast_shapes to the specification.
• follows NumPy et al in supporting an arbitrary number of input shapes to be broadcasted.
• specifies that only shapes which contain integers are explicitly supported. For shapes containing sentinel values such as None for a dimension of unknown size, behavior is left unspecified and thus implementation-defined.

[rgommers]

Thanks @kgryte! Overall LGTM and seems good to add this function to the standard. A few comments with the "unknown shape" one the key thing to discuss.

[lucascolley]

the array-api-extra docs just say

https://data-apis.org/array-api-extra/generated/array_api_extra.broadcast_shapes.html

[ev-br]

One small suggestion about the unknown dimensions, LGTM otherwise.

[lucascolley]

An issue should be opened regarding deprecation of https://data-apis.org/array-api-extra/generated/array_api_extra.broadcast_shapes.html.

[kgryte]

As all remaining comments have now been resolved and this was discussed during the last workgroup meeting, I'll go ahead and merge. Any further changes can be addressed in a follow-up PR.