Figure out what Map-like and Set-like behaviors mean

[j-f1]

The spec defines these without specific method names so we’re technically free to do anything we like (I assume they should conform to Collection or similar), although it does define how JS interprets this declaration.

https://github.com/swiftwasm/DOMKit/blob/271ef4156da79632e90ea6e41688394cdddea991/Sources/WebIDLToSwift/WebIDL%2BSwiftRepresentation.swift#L229-L243