Support variadic callbacks/closures

[j-f1]

This is currently only used by one type, TrustedTypePolicyFactory. …looking at this again, I'm not sure what exactly this needs in terms of code changes. I think it has something to do with TrustedTypePolicyOptions.

https://github.com/swiftwasm/DOMKit/blob/070f99efca195eaa38f46beeb4dc46ca1d6ce04b/Sources/WebIDLToSwift/IDLBuilder.swift#L80-L81