[red-knot] knot_extensions Python API (#15103)

## Summary

Adds a type-check-time Python API that allows us to create and
manipulate types and to test various of their properties. For example,
this can be used to write a Markdown test to make sure that `A & B` is a
subtype of `A` and `B`, but not of an unrelated class `C` (something
that requires quite a bit more code to do in Rust):
```py
from knot_extensions import Intersection, is_subtype_of, static_assert

class A: ...
class B: ...

type AB = Intersection[A, B]

static_assert(is_subtype_of(AB, A))
static_assert(is_subtype_of(AB, B))

class C: ...
static_assert(not is_subtype_of(AB, C))
```

I think this functionality is also helpful for interactive debugging
sessions, in order to query various properties of Red Knot's type
system. Which is something that otherwise requires a custom Rust unit
test, some boilerplate code and constant re-compilation.

## Test Plan

- New Markdown tests
- Tested the modified typeshed_sync workflow locally

crates/red_knot_python_semantic/src/semantic_index/definition.rs

@@ -74,6 +74,11 @@ impl<'db> Definition<'db> {
             Some(KnownModule::Typing | KnownModule::TypingExtensions)
         )
     }
+
+    pub(crate) fn is_knot_extensions_definition(self, db: &'db dyn Db) -> bool {
+        file_to_module(db, self.file(db))
+            .is_some_and(|module| module.is_known(KnownModule::KnotExtensions))
+    }
 }
 
 #[derive(Copy, Clone, Debug)]