Add Githook for Code Formatting

[EvanPiro]

Putting auto-formatting on a per commit basis would save the developer time resolving formatting issues due to failing CI.

Getting something like the following in a githook would suffice:
nix develop -c "treefmt"

[srid]

I wouldn't enable it by default, but we can add documentation (linked from README.md) explaining to the user how to do it ... possibly based on:

• https://github.com/cachix/pre-commit-hooks.nix
• https://github.com/hercules-ci/pre-commit-hooks.nix/blob/flakeModule/flake-module.nix

[srid]

Relevant: #36 (a nix check to make sure that source code was autoformatted).

[srid]

Relevant: numtide/treefmt#78

[srid]

srid/haskell-flake#136

[srid]

Closing in favour of srid/haskell-flake#136