[red-knot] Support assert_type (#15194)

## Summary

See #15103.

## Test Plan

Markdown tests and unit tests.

Author: InSyncWithFoo

crates/red_knot_python_semantic/resources/mdtest/directives/assert_type.md

@@ -0,0 +1,145 @@
+# `assert_type`
+
+## Basic
+
+```py
+from typing_extensions import assert_type
+
+def _(x: int):
+    assert_type(x, int)  # fine
+    assert_type(x, str)  # error: [type-assertion-failure]
+```
+
+## Narrowing
+
+The asserted type is checked against the inferred type, not the declared type.
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+from typing_extensions import assert_type
+
+def _(x: int | str):
+    if isinstance(x, int):
+        reveal_type(x)  # revealed: int
+        assert_type(x, int)  # fine
+```
+
+## Equivalence
+
+The actual type must match the asserted type precisely.
+
+```py
+from typing import Any, Type, Union
+from typing_extensions import assert_type
+
+# Subtype does not count
+def _(x: bool):
+    assert_type(x, int)  # error: [type-assertion-failure]
+
+def _(a: type[int], b: type[Any]):
+    assert_type(a, type[Any])  # error: [type-assertion-failure]
+    assert_type(b, type[int])  # error: [type-assertion-failure]
+
+# The expression constructing the type is not taken into account
+def _(a: type[int]):
+    assert_type(a, Type[int])  # fine
+```
+
+## Gradual types
+
+```py
+from typing import Any
+from typing_extensions import Literal, assert_type
+
+from knot_extensions import Unknown
+
+# Any and Unknown are considered equivalent
+def _(a: Unknown, b: Any):
+    reveal_type(a)  # revealed: Unknown
+    assert_type(a, Any)  # fine
+
+    reveal_type(b)  # revealed: Any
+    assert_type(b, Unknown)  # fine
+
+def _(a: type[Unknown], b: type[Any]):
+    # TODO: Should be `type[Unknown]`
+    reveal_type(a)  # revealed: @Todo(unsupported type[X] special form)
+    # TODO: Should be fine
+    assert_type(a, type[Any])  # error: [type-assertion-failure]
+
+    reveal_type(b)  # revealed: type[Any]
+    # TODO: Should be fine
+    assert_type(b, type[Unknown])  # error: [type-assertion-failure]
+```
+
+## Tuples
+
+Tuple types with the same elements are the same.
+
+```py
+from typing_extensions import assert_type
+
+from knot_extensions import Unknown
+
+def _(a: tuple[int, str, bytes]):
+    assert_type(a, tuple[int, str, bytes])  # fine
+
+    assert_type(a, tuple[int, str])  # error: [type-assertion-failure]
+    assert_type(a, tuple[int, str, bytes, None])  # error: [type-assertion-failure]
+    assert_type(a, tuple[int, bytes, str])  # error: [type-assertion-failure]
+
+def _(a: tuple[Any, ...], b: tuple[Unknown, ...]):
+    assert_type(a, tuple[Any, ...])  # fine
+    assert_type(a, tuple[Unknown, ...])  # fine
+
+    assert_type(b, tuple[Unknown, ...])  # fine
+    assert_type(b, tuple[Any, ...])  # fine
+```
+
+## Unions
+
+Unions with the same elements are the same, regardless of order.
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+from typing_extensions import assert_type
+
+def _(a: str | int):
+    assert_type(a, str | int)  # fine
+
+    # TODO: Order-independent union handling in type equivalence
+    assert_type(a, int | str)  # error: [type-assertion-failure]
+```
+
+## Intersections
+
+Intersections are the same when their positive and negative parts are respectively the same,
+regardless of order.
+
+```py
+from typing_extensions import assert_type
+
+from knot_extensions import Intersection, Not
+
+class A: ...
+class B: ...
+class C: ...
+class D: ...
+
+def _(a: A):
+    if isinstance(a, B) and not isinstance(a, C) and not isinstance(a, D):
+        reveal_type(a)  # revealed: A & B & ~C & ~D
+
+        assert_type(a, Intersection[A, B, Not[C], Not[D]])  # fine
+
+        # TODO: Order-independent intersection handling in type equivalence
+        assert_type(a, Intersection[B, A, Not[D], Not[C]])  # error: [type-assertion-failure]
+```