Package SoundSwallower as ES6 module

[dhdaines]

It seems that this is possible, so we should do it.

[dhdaines]

This should work with the latest Emscripten (see emscripten-core/emscripten#17915 (comment) for instructions)

BUT, I need to remove various uses of require from the code, which are all related to the need to run under Node.js:

• modelBase and get_model_path, which are not good APIs anyway, and should perhaps use import.meta.url somehow instead
• model file loading, for which we can use https://github.com/node-fetch/node-fetch

But see also #39 because supporting Node is not actually desireable

[dhdaines]

Or, actually, should use https://github.com/lquixada/cross-fetch

[dhdaines]

Fixed in #57