Add devshell

[gytis-ivaskevicius]

Context

Added devshell and treefmt

What does it do?

• Devshell auto loads required tooling for the project, this includes:
  • treefmt - formatter
  • just - it is unused as of right now, but im sure that it will come in handy
  • rustc
  • cargo
  • rustfmt
  • clippy
• treefmt is a simple executable that can wrap any formatter. Used to execute multiple formaters at the same time. As of right now, it supports Nix and Rust but we should consider adding formatters for yaml/markdown/etc

Testing

Tests were done by implementing CI job which validates devshell

[michalrus]

A few questions:

[michalrus]

LGTM 👍