support fields of type typing.Literal

[abrasive]

Thanks for releasing this super handy tool!

This PR does what it says on the tin, so you can have, say,

@dataclass(kw_only=True)
class UnitConfig:
    trigger: Literal["edge", "level"]

I made sure it lints clean with poetry run ruff check, was there anything else?

The docs for typing.Literal say it will take any type, but pyright and mypy will only accept bool, int, str, bytes, and enum values, so I've just implemented the first three.

Apparently mypy will also take None as one of the arguments and treat it as equivalent to Optional[Literal[. I haven't implemented this behaviour, as either Optional[] or the approach you take in the docs of using | None both work fine; will that be confusing?