Add direct support for dataclasses

[lagru]

Dataclasses require special treatment compared to normal classes when typed with a stub file. Unlike with attributes of normal classes we want to preserve whether there is a default value. So

# file.py

from dataclasses import dataclass, field, KW_ONLY, InitVar
from typing import ClassVar, Final

class Foo:
    a: float
    b: int = 3
    c: str = None
    _: KW_ONLY
    d: dict[str, Any] = field(default_factory=dict)
    e: InitVar[tuple] = tuple()
    f: ClassVar
    g: ClassVar[float]
    h: Final[ClassVar[int]] = 1

should become

# file.pyi

from dataclasses import dataclass, field, KW_ONLY, InitVar
from typing import ClassVar, Final, 

class Foo:
    a: float
    b: int = ...
    c: str = ...
    _: KW_ONLY
    d: dict[str, Any] = ...
    e: InitVar[tuple] = ...
    f: ClassVar
    g: ClassVar[float]
    h: Final[ClassVar[int]]

Note the class variable g that is still treated like a normal attribute – its value is stripped.