FS.createLazyFile in WasmFS?

`FS.createLazyFile` reads from the remote server a chunk at a time, and only on demand. This uses sync XHR, however, so it's not a good idea to use. I'm also not sure if anyone actually uses it.

For now I suggest we don't implement it in WasmFS. Though perhaps using JSPI something could be done eventually.