Switch to async counterparts for filesystem operations

[tjhorner]

I noticed that there are lots of synchronous filesystem operations (see these). This isn't really an issue since this is just an Electron app running on a computer for a single user rather than a server responding to hundreds of requests; however, it's still best practice to use the async counterparts in the fs module so it doesn't block the main event loop.

I could work on changing the code to use the async/await syntax which still keeps the code readable but frees up the event loop to do other things.