[red-knot] Type narrowing for assertions (take 2) (#17345)

## Summary

Fixes #17147.

This was landed in #17149 and then reverted in #17335 because it caused
cycle panics in checking pybind11. #17456 fixed the cause of that panic.

## Test Plan

Add new narrow/assert.md test file

Co-authored-by: Matthew Mckee <matthewmckee04@yahoo.co.uk>

crates/red_knot_python_semantic/resources/mdtest/narrow/assert.md

@@ -0,0 +1,53 @@
+# Narrowing with assert statements
+
+## `assert` a value `is None` or `is not None`
+
+```py
+def _(x: str | None, y: str | None):
+    assert x is not None
+    reveal_type(x)  # revealed: str
+    assert y is None
+    reveal_type(y)  # revealed: None
+```
+
+## `assert` a value is truthy or falsy
+
+```py
+def _(x: bool, y: bool):
+    assert x
+    reveal_type(x)  # revealed: Literal[True]
+    assert not y
+    reveal_type(y)  # revealed: Literal[False]
+```
+
+## `assert` with `is` and `==` for literals
+
+```py
+from typing import Literal
+
+def _(x: Literal[1, 2, 3], y: Literal[1, 2, 3]):
+    assert x is 2
+    reveal_type(x)  # revealed: Literal[2]
+    assert y == 2
+    reveal_type(y)  # revealed: Literal[1, 2, 3]
+```
+
+## `assert` with `isinstance`
+
+```py
+def _(x: int | str):
+    assert isinstance(x, int)
+    reveal_type(x)  # revealed: int
+```
+
+## `assert` a value `in` a tuple
+
+```py
+from typing import Literal
+
+def _(x: Literal[1, 2, 3], y: Literal[1, 2, 3]):
+    assert x in (1, 2)
+    reveal_type(x)  # revealed: Literal[1, 2]
+    assert y not in (1, 2)
+    reveal_type(y)  # revealed: Literal[3]
+```