`is_dataclass` should be a type guard and `asdict` should only work on objects with `__dataclass_fields__`

[tmke8]

@AlexWaygood proposed the following stubs in this comment: python/mypy#14263 (comment)

from typing import Any, ClassVar, Protocol
from typing_extensions import TypeGuard

class _DataclassInstance(Protocol):
 __dataclass_fields__: ClassVar[dict[str, Any]]

def asdict(obj: _DataclassInstance) -> dict[str, Any]: ...
def is_dataclass(obj: object) -> TypeGuard[_DataclassInstance]: ...

and this works as expected with dataclasses on mypy and pyright, but it doesn't currently work on pyre because pyre doesn't know yet that dataclasses have the __dataclass_fields__ classvar. (I haven't checked other type checkers.)

At runtime, is_dataclass really only checks for the __dataclass_fields__ attribute:

https://github.com/python/cpython/blob/7a0f3c1d92ef0768e082ace19d970b0ef12e7346/Lib/dataclasses.py#L1263-L1267

However, it also works with types in addition to instances, so an overload would be needed.

See also this issue for context: python/mypy#14215