Rename `pyo3::prepare_freethreaded_python` to something more meaningful

[peterjoel]

The name is confusing now that there is "free-threading" available in Python 3.13. A better name could be prepare_freestanding_python.

[davidhewitt]

I agree, I've been thinking this needs renaming for a while. I was thinking maybe pyo3::initialize_python(), or it could even go on the Python type as Python::initialize(). (The underlying C API is called Py_Initialize.)

For what it's worth, if there's any work going on to this API I'd love to also have many of the existing embedding issues explored at the same time, e.g. #1896 #1741

I suspect an API for building an initialization config would go a long way towards solving these problems.

[peterjoel]

Python::initialize() is good!

[davidhewitt]

Done in #5247. Thanks @olp-cs !