Spec: Mandate support for typing-extensions

[JelleZijlstra]

The typing spec should mandate that type checkers support the typing_extensions module by treating any name imported from it as equivalent to the same name in typing.

This is noncontroversial (I hope!), but it's still a substantive change to the spec, so it should go through the procedure in https://github.com/python/typing-council?tab=readme-ov-file#decisions. I'll likely do that in January.

[JelleZijlstra]

This was arguably addressed by the type expressions change (#1693), now at https://typing.readthedocs.io/en/latest/spec/annotations.html#type-and-annotation-expressions, which says "Most special forms must be imported from typing or typing_extensions". However, it's probably still useful to have a section mentioning typing-extensions more explicitly.