Refactor CLI + Credential storing tools + Input downloading tools

[AlisCode]

This PR implements my refactoring of the application using CLAP. 🎬

We now have 2 subcommands (cargo aoc credentials and cargo aoc input) that will get matched by CLAP and handled correctly in the AOCApp struct (app.rs), which is basically just a wrapper for the core functions, and a place to store the state of the app in, if it ever gets any (more?) complex.

Credentials are handled using the CredentialsManager struct in credentials.rs. The error handling looks good to me, at least we have cool error messages, and no unexpected crash.

Implemented an AOCDate struct in date.rs - that actually might be better off being placed in aoc_runner_internal 💁‍♀️ - It's meant to handle the day and year so we can grab the input from AoC's website.

The Credentials command stores the credentials in a file located next to the binary of cargo aoc. Potential users wouldn't like their token being accidentally exposed in their working directory, lol.

The Input command sends a GET query, setting the cookie beforehand. It then creates the file-tree and stores the result of the query in a file, thus generating input/{year}/day_{day}.txt

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Also, added a small yet cool README.md to give users a hint on how to use this wonderful tool 👍