Discover all `.nix` files in `nil diagnostics`

[9999years]

Follow-up to #125.

nil diagnostics now accepts directories in addition to files as arguments. If no arguments are given, the current directory (.) is used.

The ignore crate is used to traverse the directories and discover files. This is slightly slower than find . -path '*.nix' -exec nil diagnostics {} + because it inspects .gitignore files and performs glob matching: in my tests, this branch ran nil diagnostics in nixpkgs in 6.75 seconds, compared to 6.35 seconds for the find-based solution.