Type Narrowing: `if val == Literal` should always narrow to Literal

Feature

Currently mypy already type narrows literals in if

def works(abc: Literal["a", "b", "c"]):
    a: Literal["a"] | None = None
    if abc == "a":
        a = abc

def works_not(anystring: str):
    a: Literal["a"] | None = None
    if anystring == "a":
        a = anystring  # expression has type str

More Complete MyPy Play

Pitch

See #16813 for an example why this is useful.
In general working with TypedDicts in for loop this helps a lot reducing duplicate code / keys.

Start with the narrowing examples in the issue and reproduce them in the linked mypy-play case. Read issue #16813 for the motivating TypedDict usage, then identify the relevant narrowing implementation and tests in the repository; done means a value typed as str narrows to the compared Literal and existing narrowing behavior remains valid.