New decorator/function "@cython.dataclasses.dataclass" and "cython.dataclasses.field()" to mark dataclasses and their fields.

Tries to match the interface provided by a regular dataclass as much as possible.
This means taking the types from the dataclasses module if available (so they match exactly) or a fallback Python version that just implements the core parts (executed with "PyRun_String()" in the C source).

Use of placeholders in generated "__init__" code means the code in the C file isn't hugely readable. Probably not a huge issue, but don't really see a way round that.

As part of this I've also also implemented a Cython version of "typing.ClassVar". Although really designed for use with dataclasses it behaves sensibly when used in types in a normal cdef class. This is worth documenting more thoroughly.

Closes https://github.com/cython/cython/issues/2903