[ty] Add more tests for NamedTuples (#17975)

## Summary

Add more tests and TODOs for `NamedTuple` support, based on the typing
spec: https://typing.python.org/en/latest/spec/namedtuples.html

## Test Plan

This PR adds new tests.

crates/ty_python_semantic/resources/mdtest/named_tuple.md

@@ -37,6 +37,10 @@ Person(3, "Eve", 99, "extra")
 
 # error: [invalid-argument-type]
 Person(id="3", name="Eve")
+
+# TODO: over-writing NamedTuple fields should be an error
+alice.id = 42
+bob.age = None
 ```
 
 Alternative functional syntax:
@@ -52,6 +56,19 @@ reveal_type(alice2.id)  # revealed: @Todo(functional `NamedTuple` syntax)
 reveal_type(alice2.name)  # revealed: @Todo(functional `NamedTuple` syntax)
 ```
 
+### Definition
+
+TODO: Fields without default values should come before fields with.
+
+```py
+from typing import NamedTuple
+
+class Location(NamedTuple):
+    altitude: float = 0.0
+    latitude: float  # this should be an error
+    longitude: float
+```
+
 ### Multiple Inheritance
 
 Multiple inheritance is not supported for `NamedTuple` classes:
@@ -89,6 +106,20 @@ reveal_type(alice.level)  # revealed: int
 alice = SuperUser(1, "Alice", 3)
 ```
 
+TODO: If any fields added by the subclass conflict with those in the base class, that should be
+flagged.
+
+```py
+from typing import NamedTuple
+
+class User(NamedTuple):
+    id: int
+    name: str
+
+class SuperUser(User):
+    id: int  # this should be an error
+```
+
 ### Generic named tuples
 
 ```toml