gh-96512: Move int_max_str_digits setting to PyConfig (#96944)

It had to live as a global outside of PyConfig for stable ABI reasons in
the pre-3.12 backports.

This removes the `_Py_global_config_int_max_str_digits` and gets rid of
the equivalent field in the internal `struct _is PyInterpreterState` as
code can just use the existing nested config struct within that.

Adds tests to verify unique settings and configs in subinterpreters.