Support taking functions/closures as parameters

[j-f1]

These symbols are currently stripped from the emitted code. I think this will need generated code similar to the ClosureAttribute* types.

https://github.com/swiftwasm/DOMKit/blob/070f99efca195eaa38f46beeb4dc46ca1d6ce04b/Sources/WebIDLToSwift/IDLBuilder.swift#L49-L79

[j-f1]

Fixed in 124bd13!