add nix flake for easier dev env

[mipmip]

This Nix flake allows dev's with a Nix setup to run commands like:

## Build sc-im
nix build

## Enter a dev shell
nix develop

## Enter a shell with sc-im compiled
nix shell

## Instantly run latest version from github
nix run github:andmarti1424/sc-im

## Or run a version from someone else with a specific commit hash
nix run https://github.com/mipmip/sc-im/archive/91ba8cb61ff63e37262f6c6a96139e17c7751e35.tar.gz