js-sys: Add extends attributes for js_sys::RegExp

Part of #670

crates/js-sys/tests/wasm/RegExp.rs

@@ -1,6 +1,14 @@
 use wasm_bindgen_test::*;
+use wasm_bindgen::JsCast;
 use js_sys::*;
 
+#[wasm_bindgen_test]
+fn regexp_inheritance() {
+    let re = RegExp::new(".", "");
+    assert!(re.is_instance_of::<RegExp>());
+    assert!(re.is_instance_of::<Object>());
+}
+
 #[wasm_bindgen_test]
 fn exec() {
     let re = RegExp::new("quick\\s(brown).+?(jumps)", "ig");