* 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.