[WIP] Add support for unstable WebIDL (#1997)

* Re-enable WebGPU WebIDL as experimental

* Add `web_sys_unstable_apis` attribute

* Add test for unstable WebIDL

* Include unstable WebIDL in docs.rs builds

* Add docs and doc comment for unstable APIs

* Add unstable API checks to CI

crates/shared/src/lib.rs

@@ -94,6 +94,7 @@ macro_rules! shared_api {
             function: Function<'a>,
             method_kind: MethodKind<'a>,
             start: bool,
+            unstable_api: bool,
         }
 
         struct Enum<'a> {