Merge `_PyStackRef` definitions for `Py_GIL_DISABLED` and default builds

[colesbury]

The default build now uses _PyStackRef in a similar way to the free threaded build:

• GH-127705: Use _PyStackRefs in the default build. #127875

The implementations of most stackref functions are similar or identical, so let's combine them when it simplifies things. I think this will also make it less likely that we fix a bug in one build and miss it in the other, like happened in:

• segfault in gc with 3.14.0b1, trio and pytest #133932

I'll put up a PR soon.

Linked PRs

• gh-142472: Clean-up _PyStackRef functions #142479