## Summary
This PR adds a new `CallableTypeFromFunction` special form to allow
extracting the abstract signature of a function literal i.e., convert a
`Type::Function` into a `Type::Callable` (`CallableType::General`).
This is done to support testing the `is_gradual_equivalent_to` type
relation specifically the case we want to make sure that a function that
has parameters with no annotations and does not have a return type
annotation is gradual equivalent to `Callable[[Any, Any, ...], Any]`
where the number of parameters should match between the function literal
and callable type.
Refer
https://github.com/astral-sh/ruff/pull/16634#discussion_r1989976692
### Bikeshedding
The name `CallableTypeFromFunction` is a bit too verbose. A possibly
alternative from Carl is `CallableTypeOf` but that would be similar to
`TypeOf` albeit with a limitation that the former only accepts function
literal types and errors on other types.
Some other alternatives:
* `FunctionSignature`
* `SignatureOf` (similar issues as `TypeOf`?)
* ...
## Test Plan
Update `type_api.md` with a new section that tests this special form,
both invalid and valid forms.
08fa9b4a90