Add es6 modules.

[graphicore]

Hi, for a project I required es6 modules. I'm not sure if this fits the project scope, but it was straight forward to implement it on this base.

Usage is like:

import decompress from './wawoff2/decompress.mjs';

// src - Buffer or Uint8Array
decompress(src).then(out => {
  // store result
});

I'm available to implement changes to this PR on request.