Add an unstable C API that returns a strong reference to a single local
variable of an internal interpreter frame, addressed by its localsplus index,
with cell and free variables unboxed to their contents. Free variables are
resolved from the function closure, so the API also works on a frame that has
not started executing (before COPY_FREE_VARS runs) -- the case that motivated
it -- and it does not modify the frame.

Includes the PEP 689 deliverables: reference documentation in
Doc/c-api/frame.rst, a What's New entry for 3.16, a Misc/NEWS.d blurb, and
tests in Lib/test/test_capi/test_misc.py (TestInternalFrameApi) covering plain
locals, a cell variable, and a free variable.

Authored with the assistance of an AI coding agent (Claude Opus)