precommit hooks

[avnik]

Prototyping solution for #22

I not satisfied it much, because injecting into devshell is very hacky (or doesn't work with direnv, and require explicit nix develop at least once). Also still doubt, if we need it or not.

Open list of questions:

• May be worth to move deadnix from treefmt to pre-commit?
• May be worth run full nix flake check instead of "just treefmt"

[brainrake]

Assigning to Milestone 2 as Milestone 1 was accepted without this.

[brainrake]

I have rebased, reviewed and tested this. It did need nix develop to set up commit hooks, but they worked fine afterwards. LGTM

[brainrake]

i think nix flake check is too heavy for pre-commit.