bpo-46838: Syntax error improvements for function definitions

[pablogsal]

First round of improvements for function definitions.

https://bugs.python.org/issue46838

[pablogsal]

Thanks @AlexWaygood !!

[Andykl]

It's great that all the points for function defenition have worked out.

1. Do the new rules apply to lambda function parameters?
2. What will be for def foo(arg1, */, arg2): pass (*/, not /*)? It looks like this case is not covered.

[pablogsal]

• Do the new rules apply to lambda function parameters?

No, I will add those in another commit

What will be for def foo(arg1, /, arg2): pass (/, not /*)? It looks like this case is not covered.

I left it uncovered because that's going to be quite rare and I think is not worth the extra rule