Feat: Add a Nix flake for reproducible development environments

[MadeInShineA]

Feat: Add a Nix flake for reproducible development environments

Why this PR?

Setting up the local development environment currently requires manually installing several dependencies (DevKitPro and Xdelta3).

This PR adds a Nix flake that provides a fully reproducible dev shell with all required dependencies pre-configured for users using the Nix package manager.

What this PR does

This PR introduces 3 new files:

• flake.nix / flake.lock which defines the dev shell with typst, tinymist, just, pngquant, zopfli, and fonts
• .envrc allows to auto-entering the shell via nix-direnv on cd

And modifies the already existing build.sh file to first check if the required tools are present in the PATH, and if not, fallback to the bin directory.

The gitignore was also updated to ignore the direnv generated .direnv/ folder

How to use

# With Nix installed and flakes enabled:
nix develop

# Or with direnv + nix-direnv to enter the dev shell on directory entry:
direnv allow  # only needed once