[ty] implement TypedDict structural assignment (#21467)

Closes https://github.com/astral-sh/ty/issues/1387.

crates/ty_python_semantic/resources/mdtest/subscript/assignment_diagnostics.md

@@ -95,6 +95,7 @@ from typing import TypedDict
 
 class Person(TypedDict):
     name: str
+    phone_number: str
 
 class Animal(TypedDict):
     name: str
@@ -104,13 +105,14 @@ def _(being: Person | Animal) -> None:
     being["legs"] = 4  # error: [invalid-key]
 ```
 
-## Unknown key for all elemens of a union
+## Unknown key for all elements of a union
 
 ```py
 from typing import TypedDict
 
 class Person(TypedDict):
     name: str
+    phone_number: str
 
 class Animal(TypedDict):
     name: str