[C API] Split large object.h file: create refcount.h

[vstinner]

The Include/object.h header file became quite big (1260 lines of C code). A large part of it is just reference count management. I propose adding a new Include/refcount.h header file to ease the navigation in Include/object.h and ease the maintenance of the C API.

Linked PRs

• gh-119853: Add Include/refcount.h file  #119854
• gh-119853: Add Include/refcount.h to projects #119860

[vstinner]

I created the PR: #119854