Location information for FunctionBody should includes braces

[bakkot]

and FormalParameters should include parentheses.

This is particularly relevant if we pursue #370, but also, it's just nicer, I think.

Pros:

• Better intuition
• Adds the property that every node other than an empty Script or Module spans at least one token, simplifying some internal logic
• More consistent with other parsers

Cons:

• Does not match the ES grammar
• Arguably a breaking change
• Arguably inconsistent with grouped expressions, which do not include parentheses (and IMO should not)
• Would require making the corresponding change to shift-java's CodegenWithLocation

[michaelficarra]

Fixed by #373.