Use filesystem API to write directly into the users filesystem

[shardros]

Downloading the file each time is a good backup but is messy and anoying for the users.

The filesystem API can be used to write directly into the users file system
https://developer.mozilla.org/en-US/docs/Web/API/FileSystem

This is better than local-in-browser storage because it means the backup will be persistant even if the user clears their browser data.

We can use this to ask for filesystem permisions from the user and then we can write directly into their file system and take backups of their code that way.

#43 probally still wants to be done by downloading a zip/