🛠 Tooling: Extract file system into its own module

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

create's Creations include a files property that is a rough approximation of an in-memory file system. From https://create-josh.vercel.app/engine/runtime/creations#files:

This is the primary, most common output from blocks. Each object under files describes a folder of files. Properties whose values are strings are written as files on disk. Properties whose values are objects represent a directory.

This is a nifty file system idea. I think it could be useful in other projects. At the very least I don't want it to include create-specific ideas. I'd like to eventually spin it out into its own package(s).

Additional Info

Note to self: this isn't intended to be the most performant in-memory file system. It's meant to be the cleanest and most straightforward to use.

I'm not going to put effort into this for a while. milestone/2 Blocks Launch will happen first. Marking as blocked until then.

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