[enhancement] Nicer error handling / more support for arguments

[henryiii]

There are several possible ways to improve argument handling, but that probably doesn't have to be in this PR. Ideas:

• (a, *args, b=None) is valid in Python but not pybind11.
• (a, /, **kwargs) allows "a=" to go into kwargs
• (a, / a) isn't valid in Python, but we could allow it
• We could check to make sure py::pos_only() is not at the beginning and py::kw_only() is not at the end.
• We could have a nice error if either was given twice or in the wrong order.

Originally posted by @henryiii in #2459 (comment)