feat: add Nix support for reproducible builds

[willyrgf]

Packaging the repository apps with Nix enables people to easily run these apps and extend them for other workflows.
A few tests pointing to my fork and commit (correct path if merged is in committed README.md):

# running gdformat
nix run 'github:willyrgf/godot-gdscript-toolkit/4fe2a357831130d774f74942809577e7408f6742#gdformat' script.gd
0 files reformatted, 1 file left unchanged.

# running default app will show a helper
❯ nix run 'github:willyrgf/godot-gdscript-toolkit/4fe2a357831130d774f74942809577e7408f6742'

GDScript Toolkit - Available commands:

Usage: nix run github:Scony/godot-gdscript-toolkit#<command> -- [args]

Available commands:
  gdparse   - GDScript parser for debugging and educational purposes
  gdlint    - GDScript linter that performs static analysis
  gdformat  - GDScript formatter that formats code according to predefined rules
  gd2py     - GDScript to Python transpiler
  gdradon   - GDScript code complexity analysis (calculates cyclomatic complexity)

Examples:
  nix run github:Scony/godot-gdscript-toolkit#gdformat -- --help
  nix run github:Scony/godot-gdscript-toolkit#gdlint -- myfile.gd
  nix run github:Scony/godot-gdscript-toolkit#gdparse -- myfile.gd -p

For local development:
  nix develop  # Enter development shell with all tools available

# all apps for --all-systems
❯ nix flake show 'github:willyrgf/godot-gdscript-toolkit/4fe2a357831130d774f74942809577e7408f6742' --all-systems
github:willyrgf/godot-gdscript-toolkit/4fe2a357831130d774f74942809577e7408f6742?narHash=sha256-rtyKi7OtZrxfXUw55Gu8i5nvDGSPBH1Ycz5FAXJpThg%3D
├───apps
│   ├───aarch64-darwin
│   │   ├───default: app: Show available GDScript Toolkit commands
│   │   ├───gdformat: app: GDScript formatter
│   │   ├───gdlint: app: GDScript linter
│   │   ├───gdparse: app: GDScript parser
│   │   ├───gdradon: app: GDScript code complexity analysis
│   │   └───help: app: Show available GDScript Toolkit commands
│   ├───aarch64-linux
│   │   ├───default: app: Show available GDScript Toolkit commands
│   │   ├───gdformat: app: GDScript formatter
│   │   ├───gdlint: app: GDScript linter
│   │   ├───gdparse: app: GDScript parser
│   │   ├───gdradon: app: GDScript code complexity analysis
│   │   └───help: app: Show available GDScript Toolkit commands
│   ├───x86_64-darwin
│   │   ├───default: app: Show available GDScript Toolkit commands
│   │   ├───gdformat: app: GDScript formatter
│   │   ├───gdlint: app: GDScript linter
│   │   ├───gdparse: app: GDScript parser
│   │   ├───gdradon: app: GDScript code complexity analysis
│   │   └───help: app: Show available GDScript Toolkit commands
│   └───x86_64-linux
│       ├───default: app: Show available GDScript Toolkit commands
│       ├───gdformat: app: GDScript formatter
│       ├───gdlint: app: GDScript linter
│       ├───gdparse: app: GDScript parser
│       ├───gdradon: app: GDScript code complexity analysis
│       └───help: app: Show available GDScript Toolkit commands
├───devShells
│   ├───aarch64-darwin
│   │   └───default: development environment 'gdtoolkit-dev-env'
│   ├───aarch64-linux
│   │   └───default: development environment 'gdtoolkit-dev-env'
│   ├───x86_64-darwin
│   │   └───default: development environment 'gdtoolkit-dev-env'
│   └───x86_64-linux
│       └───default: development environment 'gdtoolkit-dev-env'
└───packages
    ├───aarch64-darwin
    │   ├───default: package 'python3.13-gdtoolkit-4.3.4'
    │   └───gdtoolkit: package 'python3.13-gdtoolkit-4.3.4'
    ├───aarch64-linux
    │   ├───default: package 'python3.13-gdtoolkit-4.3.4'
    │   └───gdtoolkit: package 'python3.13-gdtoolkit-4.3.4'
    ├───x86_64-darwin
    │   ├───default: package 'python3.13-gdtoolkit-4.3.4'
    │   └───gdtoolkit: package 'python3.13-gdtoolkit-4.3.4'
    └───x86_64-linux
        ├───default: package 'python3.13-gdtoolkit-4.3.4'
        └───gdtoolkit: package 'python3.13-gdtoolkit-4.3.4'

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Really useful apps btw, thank you for all your work.