[js] provide native RegExp extern and use it in EReg

This adds an extern for native js `RegExp` class as well as change `EReg` implementation to use that instead of `Dynamic`.

The match returned by `RegExp.exec` is an extended `Array` with `index` and `input` fields, so I had to weaken basic class extending restriction for extern classes.

This extern is required for proper interoperability with native JS, for example node.js `assert.throws` accepts native javascript `RegExp`s: http://nodejs.org/api/assert.html#assert_assert_throws_block_error_message