✨ Added new Error to TypedDict

See: [python#4617](python#4617)

This allows the following code to trigger the error
`typeddict-unknown-key`

```python
A = T.TypedDict("A", {"x": int})

def f(x: A) -> None:
    ...

f({"x": 1, "y": "foo"})
```

The user can then safely ignore this specific error at their
disgression.

mypy/errorcodes.py

@@ -67,6 +67,9 @@ def __str__(self) -> str:
 TYPEDDICT_ITEM: Final = ErrorCode(
     "typeddict-item", "Check items when constructing TypedDict", "General"
 )
+TYPPEDICT_UNKNOWN_KEY: Final = ErrorCode(
+    "typeddict-unknown-key", "Check unknown keys when constructing TypedDict", "General"
+)
 HAS_TYPE: Final = ErrorCode(
     "has-type", "Check that type of reference can be determined", "General"
 )

mypy/messages.py

@@ -1633,7 +1633,7 @@ def unexpected_typeddict_keys(
                             format_key_list(extra, short=True), format_type(typ)
                         ),
                         context,
-                        code=codes.TYPEDDICT_ITEM,
+                        code=codes.TYPPEDICT_UNKNOWN_KEY,
                     )
                     return
         found = format_key_list(actual_keys, short=True)