Rename module_systems 'es6' to 'esm'

Currently melange, via dune, can be configured to compile to common js (cjs) OR to EcmaScript Modules (esm). Right now the option incorrectly uses the name es6 to output esm. ES6, also known as EcmaScript 2015, is a versioned specification of the EcmaScript language (https://262.ecma-international.org/6.0/).

We should update Dune and the Melange documentation to reflect this and use esm instead of es6