wasm: make qtloader.js use FS.createPreloadedFile when preloading

qtloader.js currently fetches and copies the preload files manually. Emscripten provides FS.createPreloadedFile for this use case, can we use that function instead?

The main benefit of this (besides removing removing code from qtloader.js), is that Emscripten runs "preload plugins" for preloaded files. In the case of .so shared object files this will compile the .so to a wasm module ahead of time, which avoids the restrictions that some browsers have on compiling wasm modules synchronously.