Add a directory of filesystem templates

[jvilk]

This will make it even easier to make a custom BrowserFS file system (and I'd use them, too).

I should start by making templates for the following cases:

• Read-only filesystems
• Read/write filesystems

[perimosocordiae]

Or even abstract FS implementations that you can extend, or helper functions even. There's a lot of shared code between the localStorage and in-memory FSes.

[jvilk]

Yeah. I figured that I might do that once I have a third working filesystem in-place.

[jvilk]

No need to do this anymore, since TypeScript checks interfaces at compile-time.