PEP 728: Revise proposal to spec the 'extra_items=T' idea (python#4066)

- Move away from '__extra_items__: T' to 'extra_items=T'

- Repurpose the 'closed' class parameter to make 'closed=True'
  an alias of 'extra_items=Never'

- Include references to the typing spec

- Normalize terms like "assignability" and "consistent" to match the
  typing glossary (https://typing.readthedocs.io/en/latest/spec/glossary.html)

- Update sections that contain documentation quotes to match the
  up-to-date version

- Add new sections about other ideas

- Bump the target version to 3.14

Signed-off-by: Zixuan James Li <[email protected]>
Co-authored-by: Jelle Zijlstra <[email protected]>