gh-59956: Allow the "Trashcan" Mechanism to Work Without a Thread State

[ericsnowcurrently]

We've factored out a struct from the two PyThreadState fields. This accomplishes two things:

• make it clear that the trashcan-related code doesn't need any other parts of PyThreadState
• allows us to use the trashcan mechanism even when there isn't a "current" thread state

We still expect the caller to hold the GIL.

• Issue: [subinterpreters] Make the PyGILState API compatible with subinterpreters #59956