[3.12] gh-105716: Support Background Threads in Subinterpreters Consistently (gh-109921) (gh-110707)

The existence of background threads running on a subinterpreter was preventing interpreters from getting properly destroyed, as well as impacting the ability to run the interpreter again. It also affected how we wait for non-daemon threads to finish.

We add PyInterpreterState.threads.main, with some internal C-API functions.

(cherry-picked from commit 1dd9dee45d2591b4e701039d1673282380696849)