Improve support for bundlers

[delvedor]

It's becoming a common pattern the use of bundlers in serverless environments, and the client does not support them officially and might have some compatibility issues due to its lazy load nature.

Webpack should be supported thanks to #783, but there are no tests for verifying it nor documentation, while Parcel and Rollup have never been checked.
We'll work towards providing better support for bundlers and create a reliable test infrastructure.

If you have a specific use case you would like to see supported, or want to provide some insight, feel free to share it here!

Related: #781