Implement Default for JS types

[Jules-Bertholet]

• Implement the Default trait for types in js-sys where it makes sense
• Generate a Default impl for WebIDL types with zero-argument constructors

[alexcrichton]

Thanks for this!

I think the 0-argument ctor makes sense for web-sys, although some of the default implementations in js-sys seem like they're a bit ambitiuos, e.g. creating an error without a string and such. Could the initial set of ctors in js-sys be a bit more conservative to basically match Default semantics in Rust? (e.g. errors typically don't implement Default)

[Jules-Bertholet]

Ready for review