chore: convert flake to flake parts and add git-hooks (#1599)

This PR adds

- flake-parts to the flake
- a git hook that checks formatting on push
- npm package installation if node_modules does not exist to prevent
errors when pushing

The reason I set on-push for the hooks is, that people could get quite
annoyed when having to check for proper formatting on every commit.
Since we doe merge PRs anyway, we can require proper formatting on push
and ease local development for contributors.

I'd like to hear feedback from other contributors here on what to add or
to change before I merge this into main.

.gitignore

@@ -9,3 +9,5 @@
 /public/guides/nix-pills/
 /public/demos/*.cast
 /public/demos/*.svg
+
+.pre-commit-config.yaml