[red-knot] Add assignability of function literals to callables (#17095)

## Summary

Part  of #16953

## Test Plan

Update is_assignable_to.md

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

@@ -476,4 +476,21 @@ static_assert(is_assignable_to(CallableTypeOf[keyword_variadic], Callable[..., N
 static_assert(is_assignable_to(CallableTypeOf[mixed], Callable[..., None]))
 ```
 
+### Function types
+
+```py
+from typing import Any, Callable
+
+def f(x: Any) -> str:
+    return ""
+
+def g(x: Any) -> int:
+    return 1
+
+c: Callable[[Any], str] = f
+
+# error: [invalid-assignment] "Object of type `Literal[g]` is not assignable to `(Any, /) -> str`"
+c: Callable[[Any], str] = g
+```
+
 [typing documentation]: https://typing.readthedocs.io/en/latest/spec/concepts.html#the-assignable-to-or-consistent-subtyping-relation