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 implements callback interfaces for WebIDL, the final WebIDL
construct that we were unconditionally ignoring! Callback interfaces are
implemented as dictionaries of callbacks. Single-operation callback interfaces
are also expanded when flattening to accept a `Function` as well, in accordance
with the WebIDL spec.
New features have been added to `web-sys` for all the new callback interface
types. Additionally the `EventTarget.webidl` was tweaked to not have
`EventListener?` as this is required for all functional usage and there's no
need to keep that sort of web browser compat here.
Closes#258
