[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/module_resolver/module.rs

@@ -109,6 +109,7 @@ pub enum KnownModule {
     #[allow(dead_code)]
     Abc, // currently only used in tests
     Collections,
+    KnotExtensions,
 }
 
 impl KnownModule {
@@ -122,6 +123,7 @@ impl KnownModule {
             Self::Sys => "sys",
             Self::Abc => "abc",
             Self::Collections => "collections",
+            Self::KnotExtensions => "knot_extensions",
         }
     }
 
@@ -147,6 +149,7 @@ impl KnownModule {
             "sys" => Some(Self::Sys),
             "abc" => Some(Self::Abc),
             "collections" => Some(Self::Collections),
+            "knot_extensions" => Some(Self::KnotExtensions),
             _ => None,
         }
     }
@@ -154,4 +157,8 @@ impl KnownModule {
     pub const fn is_typing(self) -> bool {
         matches!(self, Self::Typing)
     }
+
+    pub const fn is_knot_extensions(self) -> bool {
+        matches!(self, Self::KnotExtensions)
+    }
 }