Adding File Ignoring (MfGames Project Setup Flake)

While using this flake, it became obvious that there were a few things that were still rough around the edges. In specific, we didn't add /.direnv/ to the .gitignore file sof if we weren't careful, we would be adding the entire environment into the check-in.

The other was that some of the generated files ( in specific) would be added via Nixago, then formatted as part of our treefmt call in lefthook, then checked-in. As soon as someone went into the environment, Nixago would notice that the file was different (because of formatting) and would regenerate it, leading to an endless loop of generation/formatting.

In both cases, we needed the ability to add files to various ignore files (.gitignore in the first case and .prettierignore in the second). The v0.1.0 check-in does that, and also avoids reformatting the license files along the way.