Allow using TypedDict for more precise typing of **kwds (#13471)
Fixes #4441
This uses a different approach than the initial attempt, but I re-used some of the test cases from the older PR. The initial idea was to eagerly expand the signature of the function during semantic analysis, but it didn't work well with fine-grained mode and also mypy in general relies on function definition and its type being consistent (and rewriting `FuncDef` sounds too sketchy). So instead I add a boolean flag to `CallableType` to indicate whether type of `**kwargs` is each item type or the "packed" type.
I also add few helpers and safety net in form of a `NewType()`, but in general I am surprised how few places needed normalizing the signatures (because most relevant code paths go through `check_callable_call()` and/or `is_callable_compatible()`). Currently `Unpack[...]` is hidden behind `--enable-incomplete-features`, so this will be too, but IMO this part is 99% complete (you can see even some more exotic use cases like generic TypedDicts and callback protocols in test cases).
