[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/webidl-tests/unstable.rs

@@ -0,0 +1,17 @@
+use wasm_bindgen_test::*;
+
+include!(concat!(env!("OUT_DIR"), "/unstable.rs"));
+
+#[cfg(web_sys_unstable_apis)]
+#[wasm_bindgen_test]
+fn can_use_unstable_apis() {
+    let unstable_interface = GetUnstableInterface::get();
+    assert_eq!(0u32, unstable_interface.enum_value());
+
+    let mut dict = UnstableDictionary::new();
+    dict.unstable_enum(UnstableEnum::B);
+    assert_eq!(
+        2u32,
+        unstable_interface.enum_value_with_unstable_dictionary(&dict)
+    );
+}