feat: small command line helpers for quick tests

[henryiii]

Description

These are some small command line helpers intended for quick testing, based on python-config.
On a Unix system, it should be possible to do:

c++ pybindclass.cpp $(pybind11-config --cflags) $(pybind11-config --ldflags) -o pybindclass.$(python-config --extension-suffix)

Or even:

c++ $(pybind11-config --file=pybindclass.cpp)

This is handy for quick tests on issue reports.

Since I have to debug embedded code too, I also added support for --embed:

c++ $(pybind11-config --embed --file=pybindclass.cpp)

Based on https://github.com/python/cpython/blob/3.11/Misc/python-config.in

Suggested changelog entry:

* Small extra features added to the pybind11 command line tool based on python-config.

I can probably come up with a test or two.