[red-knot] support typing.Union in type annotations (#14499)

Fix #14498

## Summary

This PR adds `typing.Union` support

## Test Plan

I created new tests in mdtest.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/annotations/union.md

@@ -0,0 +1,61 @@
+# Union
+
+## Annotation
+
+`typing.Union` can be used to construct union types same as `|` operator.
+
+```py
+from typing import Union
+
+a: Union[int, str]
+a1: Union[int, bool]
+a2: Union[int, Union[float, str]]
+a3: Union[int, None]
+a4: Union[Union[float, str]]
+a5: Union[int]
+a6: Union[()]
+
+def f():
+    # revealed: int | str
+    reveal_type(a)
+    # Since bool is a subtype of int we simplify to int here. But we do allow assigning boolean values (see below).
+    # revealed: int
+    reveal_type(a1)
+    # revealed: int | float | str
+    reveal_type(a2)
+    # revealed: int | None
+    reveal_type(a3)
+    # revealed: float | str
+    reveal_type(a4)
+    # revealed: int
+    reveal_type(a5)
+    # revealed: Never
+    reveal_type(a6)
+```
+
+## Assignment
+
+```py
+from typing import Union
+
+a: Union[int, str]
+a = 1
+a = ""
+a1: Union[int, bool]
+a1 = 1
+a1 = True
+# error: [invalid-assignment] "Object of type `Literal[b""]` is not assignable to `int | str`"
+a = b""
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import Union
+
+a: Union[int, str]
+
+def f():
+    # revealed: int | str
+    reveal_type(a)
+```