FileSystem options/integration documentation

Related to: https://github.com/isomorphic-git/lightning-fs/issues/117

I'm currently trying to see about using [isomorphic-git](https://github.com/isomorphic-git/isomorphic-git) with [lightning-fs](https://github.com/isomorphic-git/lightning-fs) and then use an emscripten-built assembler to output a file and then eventually call a linker.

However, I'm having trouble figuring out how exactly to go about interfacing lightning-fs's asynchronous nature with Emscripten's synchronous one.

I believe I can bridge the two by copying data from lightning-fs in memory to Emscripten's FS and going back and forth, but there has to be a better way... I'd like to ask for some guidance and possibly updated documentation with integrating an external asynchronous FS with Emscripten's, as I think it may not be that uncommon a use-case.

I also can't seem to find much documentation how to use WasmFS, though I've read it's partially implemented and may be sufficient even for this use-case.