Try again with proper build

[tomayac]

@connorjclark This is a second attempt. I guess I simply don't get what to do with Module. I tried to follow this example, but not sure it's applicable.

The main idea is to get rid of this hack, where I append export { potrace, ready as init }; to the output: https://github.com/tomayac/esm-potrace-wasm/blob/7d81a9c19ed74fe135107a9328b7adc58a5b8f57/package.json#L25

The proper way to get a module seems to be -s "MODULARIZE=1" -s "EXPORT_ES6=1".