make helpers available in the browser

[disnet]

We need to extend the browser loader to allow the editor to import the helper library. ie this needs to work:

import { isIdentifier} from 'sweet.js/helpers' for syntax;

syntax m = ctx => {
  if (isIdentifer(ctx.next().value)) {
    return #`true`;
  }
  return #`false`;
}

[gabejohnson]

Fixed by #671