Caching docs in Deno.Kv and testable file system helpers

[taras]

Motivation

I started rewriting the GitHub Actions workflows from #891 into Deno tasks. In the process, I realized that I would like to cache the contents of the docs directory so I can load it from Deno.Kv instead of untarballing the files in the Deno instance. As it was getting more complicated, I wanted to start building up helpers and ensure they were tested. I kept ballooning the requirements, but it's good practice.

Approach

1. Created useCwd hook that uses CwdContext for storing the current context.
2. Created useReadDir and useMkdir that are useCwd compatible
3. Started writing test

TODO

1. My useCwd doesn't actually work, it's something about the way I'm using scope. useReadDir and useMkdir don't actually get the set value.