Define order of operations in derivation expressions

[tokoko]

This is a continuation of a discussion from slack. Extensions function return type expressions allow expressing types with math operators as defined in the spec, but the following points aren't clear:

• Are parenthesis allowed? - spec doesn't say one way or the other but antlr grammar in substrait-java seems to assume they are.
• Should evaluation follow the usual order of operations (pemdas)? - once again, spec is unclear, while antlr grammar doesn't follow pemdas, or to be exact it treats parenthesis with the highest precedence, but doesn't differentiate between multiplication/division and addition/subtraction.

I think we should both allow parenthesis and follow pemdas as that is what's most intuitive in the context. One way or the other, this should be made clear in the spec.