Support for Partial types

Feature

File t.py:

from typing_extensions import TypedDict


class Foo(TypedDict):
    x:int
    y:int


foo: Partial[Foo] = {'x': 1}
# reveal_type(foo) -> TypedDict('t.Foo?', {'x'?: builtins.int, 'y'?: builtins.int})

Pitch

For some uses cases, e.g. processing PATCH requests on a web server, we usually want to allow a partial version of the original model. Currently this must be done manually, and that can be a bit error prone since one may forget to update the partial model. The Partial type annotation would transform the original type into its optional version.

Related to: https://github.com/python/mypy/issues/4128
Note: Examples taken from https://stackoverflow.com/questions/69091758/python-typehint-subsetpartial-of-an-typeddict

Start with the t.py example and the related issue #4128 to understand the proposed Partial behavior for TypedDict. Done means the annotation accepts partial Foo values and reveal_type(foo) shows optional x and y fields as specified.