Rework of arg/return type hints to support .noconvert()

[timohl]

Description

While working on improving type hints for Eigen/numpy, I found that with the current solution from #5450 it is not possible to have the type hints be influenced by the py::arg().noconvert() option.
Certain Eigen types offer to be loaded from anything np.array() accepts which would fit to np.typing.ArrayLike.
With .noconvert() however, only instances of numpy arrays are accepted, which would fit np.typing.NDArray[<dtype>].
It would be nice if .noconvert() would force using the return type (which is the more specific type).

Meanwhile, I found that nanobind implements a similar arg/return type hint system using io_name.
This system inserts @arg-type@return-type@ using @ as a special character, which is evaluated in a later signature parsing stage.
At this stage, the .noconvert() info is available, and I was able to implement it similarly.

Additionally, I added arg_descr and return_descr to force using arg or return type hints.
This is useful for correctly type hinting typing::Callable.
The current implementation in this PR does not allow nested Callables (e.g., Callable that has a Callable as argument), but I will add support for that later this week (requires using a stack container to track nested arg_descr/return_descr).
(See 077d0b6)

Finally, I found bugs when using typing::Literal with certain special characters:
Currently, %, {, } fail with ImportError: Internal error while parsing type signature (2).
With this PR, the added special character @ fails as well when used in a Literal.
I will try to find a solution for that later this week as well.
(See 9877aca)

This also fixes #5477 by using pathlib.Path instead of Path.
(See 9d0766c)

Suggested changelog entry:

Rework of arg/return type hints to support .noconvert()

Open issues:

• Nested arg_descr/return_descr do not work (require tracking using a Stack)
• Special characters in typing.Literal fail (%, {, }, @)
• Fix usage of auto return type deduction (not supported in C++11)
• Fix clang-tidy issues

Relevant nanobind code

This PR is based on the following code in nanobind:

https://github.com/wjakob/nanobind/blob/698f449a207e321b364278ff68f0b161127fd3a0/include/nanobind/nb_descr.h#L81-L86

https://github.com/wjakob/nanobind/blob/698f449a207e321b364278ff68f0b161127fd3a0/src/nb_func.cpp#L1018-L1234

[timohl]

Checks using C++11 fail because I used auto return type deduction which is only supported since C++14.
Will fix that soon.