Create bindings for RegExp (#580)

* Create bindings for RegExp

* Address review comments

- Split the constructor into two: `new` and `new_regexp`. This way we
  can write RegExp::new("foo", "g") rather than
  RegExp::new(&JsValue::from("foo"), "g").

- The js_name for the setter for lastIndex should be `lastIndex` and
  not `set_lastIndex`. But fixing this causes a panic. Remove the
  method for now.

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

@@ -25,6 +25,7 @@ pub mod Number;
 pub mod Object;
 pub mod Proxy;
 pub mod Reflect;
+pub mod RegExp;
 pub mod Set;
 pub mod SetIterator;
 pub mod Symbol;