This information is embedded within the algorithm for constructing interfaces
and their prototypes in the section for ECMAScript glue in the WebIDL spec...
This really *should* make the `wasm_bindgen_backend::ast::ImportType::extends`
member from a `Vec<Ident>` into a `Vec<syn::Path>` so that we could use
`js_sys::Object` in the extends field, but that is a huge pain because then the
`ImportedTypes` trait needs to be changed, and all of its implementers, etc...
This commit adds support for the WebIDL `Callback` type by translating all
callbacks to the `js_sys::Function` type. This will enable passing raw JS values
into callbacks as well as Rust valus using the `Closure` type.
This commit doesn't currently implement "callback interfaces" in WebIDL, that's
left for a follow-up commit.
