fix(cmake): support DEBUG_POSTFIX correctly (#4761)

* cmake: split extension

Into suffix and debug postfix. Pybind11 is currently treating both as
suffix, which is problematic when something else defines the
DEBUG_POSTFIX because they will be concatenated.

pybind11_extension sets SUFFIX to _d.something and if DEBUG_POSTFIX is
set to _d.

    _d + _d.something = _d_d.something

The issue has been reported at:

https://github.com/pybind/pybind11/issues/4699



* style: pre-commit fixes

* fix(cmake): support postfix for old FindPythonInterp mode too

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>