Introduce bundler or export an es module

Bundle.run has been discontinued, and for this reason the [browser module](https://bundle.run/geojson-rewind) suggested in the README doesn't work anymore.

Please introduce some bundler like browserify or rollup, or even better, convert this into an es module, since now [node supports those as well](https://nodejs.org/api/esm.html#esm_package_json_type_field).

Start with the README section that links to the discontinued Bundle.run browser module, then inspect the package's current entry points and distribution setup. Decide whether bundling or native ES module export fits the project and its supported Node versions. Done means the browser usage described in the README works without Bundle.run and the chosen module format is documented.

Written by the indexing model from the issue text.