Emscripten: Add option for Modern ES6 modules

[dukesook]

The -sEXPORT_ES6 flag adds the following export statement to libheif.js:

export default libheif;

This allows libheif.js to be imported with the modern import keyword.

I also believe this setting should be on by default since ESM is the standard going forward.
The test case in test-javascript.js uses require() instead of import.

[farindk]

Yes, I think this should be explicitly enabled (with USE_ES6) because it appears to be not backwards compatible. If I see this correctly, EXPORT_ES6 also enables MODULARIZE, which changes how the functions are exported.