gh-76785: Improved Subinterpreters Compatibility with 3.12 (gh-115424)

For the most part, these changes make is substantially easier to backport subinterpreter-related code to 3.12, especially the related modules (e.g. _xxsubinterpreters). The main motivation is to support releasing a PyPI package with the 3.13 capabilities compiled for 3.12.

A lot of the changes here involve either hiding details behind macros/functions or splitting up some files.