Consider `auto-initialize` feature for `pyembed`

An idea suggested in #500 was to have a crate feature in `pyembed` for auto-initializing the Python interpreter. Kind of like what the `cpython` and `pyo3` crates do so there is a turnkey way for the crate itself to manage a global interpreter state without consumers having to write much code.

My knee jerk reaction is the idea is worth pursuing.

Start by reviewing issue #500 and the existing pyembed crate, then compare how the cpython and pyo3 crates handle automatic Python interpreter initialization. Establish the feature's scope, global interpreter-state behavior, and consumer-facing interface before deciding whether the proposal is ready for implementation.

Written by the indexing model from the issue text.