[red-knot] Handle special case returning NotImplemented (#17034)

## Summary

Closes #16661

This PR includes two changes:

- `NotImplementedType` is now a member of `KnownClass`
- We skip `is_assignable_to` checks for `NotImplemented` when checking
return types

### Limitation

```py
def f(cond: bool) -> int:
    return 1 if cond else NotImplemented
```

The implementation covers cases where `NotImplemented` appears inside a
`Union`.
However, for more complex types (ex. `Intersection`) it will not worked.
In my opinion, supporting such complexity is unnecessary at this point.

## Test Plan

Two `mdtest` files were updated:

- `mdtest/function/return_type.md`
- `mdtest/type_properties/is_singleton.md`

To test `KnownClass`, run:
```bash
cargo test -p red_knot_python_semantic -- types::class::
```

crates/red_knot_python_semantic/resources/mdtest/function/return_type.md

@@ -269,3 +269,45 @@ def f(cond: bool) -> int:
     if cond:
         return 2
 ```
+
+## NotImplemented
+
+`NotImplemented` is a special symbol in Python. It is commonly used to control the fallback behavior
+of special dunder methods. You can find more details in the
+[documentation](https://docs.python.org/3/library/numbers.html#implementing-the-arithmetic-operations).
+
+```py
+from __future__ import annotations
+
+class A:
+    def __add__(self, o: A) -> A:
+        return NotImplemented
+```
+
+However, as shown below, `NotImplemented` should not cause issues with the declared return type.
+
+```py
+def f() -> int:
+    return NotImplemented
+
+def f(cond: bool) -> int:
+    if cond:
+        return 1
+    else:
+        return NotImplemented
+
+def f(x: int) -> int | str:
+    if x < 0:
+        return -1
+    elif x == 0:
+        return NotImplemented
+    else:
+        return "test"
+
+def f(cond: bool) -> str:
+    return "hello" if cond else NotImplemented
+
+def f(cond: bool) -> int:
+    # error: [invalid-return-type] "Object of type `Literal["hello"]` is not assignable to return type `int`"
+    return "hello" if cond else NotImplemented
+```