Parser should allow arbitrary newlines inside of any bracket type

[sbillig]

What is wrong?

The parser allows any newlines inside of parentheses, but not inside square, angle, or curly brackets (unless they happen to be inside of parentheses 😅). The primary issue right now is that list expressions can't contain newlines.

(I could be convinced that newlines shouldn't be allowed in angle brackets, btw; I don't really care either way.)

How can it be fixed

The logic inside parser.rs that touches the paren_stack should be extended to all bracket types.

[sbillig]

This was probably implemented as part of #547, but I'm not in the mood to verify right now.