Fix some issues when launching threaded cores

[white-axe]

Support for threaded cores was added in #75, but I think it can be improved.

• If the JavaScript file and WebAssembly file for the core are resolvable as blobs, we can automate the process of loading the core without having the user manually set the hooks as specified in Support for multi threading cores #75. I have modified setupEmscripten() to do this. Please note the core needs to have been built with at least RetroArch 1.21.0 and Emscripten 4.0.8 (I believe the extra .worker.js file that comes with threaded cores disappeared around RetroArch 1.21.0).
• If RetroArch is built with -DHAVE_WASMFS=1 and/or -DHAVE_EXTRA_WASMFS=1, the latter of which is currently required by the threaded version of the RetroArch web player, there will not be a Module.FS.init() function, so the code shouldn't assume it exists.
• mainScriptUrlOrBlob can also be a blob.

[arianrhodsandlot]

Thanks!