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.

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📚 Documentation preview 📚: https://pybind11--4272.org.readthedocs.build/

[henryiii]

🤖 AI text below 🤖

Rebased on current master as a single commit. Changes beyond the rebase:

• Fixed a LIBIDR → LIBDIR typo in the embed link flags.
• Fixed the Py_ENABLE_SHARED condition: python-config adds -L$LIBPL for static builds, not shared ones. Verified by compiling and running an embedded program against a static (Homebrew framework) Python.
• -std=c++11 → -std=c++17 (v3 minimum), and it is now always added, not only when CFLAGS is set.
• --extension is now --file with --embed support, matching the final design in the PR description; the file path keeps its directory and output paths are quoted like the existing flags.
• Kept master's --extension-suffix (added separately in the meantime) and shared its include-dir logic via a new get_include_dirs().
• Added tests in tests/extra_python_package/test_files.py and a short section in docs/compiling.rst.