webidl: All interfaces implicitly extend Object

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...

crates/webidl-tests/array.rs

@@ -1,3 +1,4 @@
+use js_sys::Object;
 use wasm_bindgen_test::*;
 
 include!(concat!(env!("OUT_DIR"), "/array.rs"));