Library dxt-js as optional dependency

[Timic3]

RW parser as a whole has quite large bundle size, considering 87% of the size comes from dxt-js library. We could implement DXT decompression (we don't need compression) functionality in JavaScript/TypeScript and ship significantly smaller bundle size with no dependencies whatsoever. It might be a bit slower, but if people want to trade speed for smaller bundle size, this should be an option.