Implement experimental support for PEP 728

[erictraut]

Draft PEP 728 proposes to add a way to specify the type of "extra fields" within a TypedDict.

It would be helpful for pyright to implement provisional support for this as a way to help validate the technical design and usability of the proposal.

[erictraut]

This will be included in the next release of pyright.

[erictraut]

This is addressed in pyright 1.1.352, which I just published.