Implement support for WebIDL Callback types

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.

crates/webidl/src/lib.rs

@@ -138,7 +138,7 @@ fn builtin_idents() -> BTreeSet<Ident> {
         vec![
             "str", "char", "bool", "JsValue", "u8", "i8", "u16", "i16", "u32", "i32", "u64", "i64",
             "usize", "isize", "f32", "f64", "Result", "String", "Vec", "Option",
-            "ArrayBuffer", "Object", "Promise",
+            "ArrayBuffer", "Object", "Promise", "Function",
         ].into_iter()
             .map(|id| proc_macro2::Ident::new(id, proc_macro2::Span::call_site())),
     )