[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

crates/red_knot_python_semantic/src/types.rs

@@ -900,6 +900,12 @@ impl<'db> Type<'db> {
                 Type::Callable(CallableType::General(target_callable)),
             ) => self_callable.is_assignable_to(db, target_callable),
 
+            (Type::FunctionLiteral(self_function_literal), Type::Callable(_)) => {
+                self_function_literal
+                    .into_callable_type(db)
+                    .is_assignable_to(db, target)
+            }
+
             // TODO other types containing gradual forms (e.g. generics containing Any/Unknown)
             _ => self.is_subtype_of(db, target),
         }