refactor(scripts): move to scripts.nix

[r17x]

Features:

• overlay dvt.overlay.default
• expose scripts in packages

❯ nix flake show
├───devShells
│   ├───aarch64-darwin
│   │   └───default: development environment 'nix-shell'
│   ├───aarch64-linux
│   │   └───default: development environment 'nix-shell'
│   ├───x86_64-darwin
│   │   └───default: development environment 'nix-shell'
│   └───x86_64-linux
│       └───default: development environment 'nix-shell'
├───overlay: Nixpkgs overlay
├───packages
│   ├───aarch64-darwin
│   │   ├───dvt-format: package 'format'
│   │   ├───dvt-init: package 'dvt'
│   │   ├───dvt-update: package 'update'
│   │   └───test-develop: package 'test-develop'
│   ├───aarch64-linux
│   │   ├───dvt-format: package 'format'
│   │   ├───dvt-init: package 'dvt'
│   │   ├───dvt-update: package 'update'
│   │   └───test-develop: package 'test-develop'
│   ├───x86_64-darwin
│   │   ├───dvt-format: package 'format'
│   │   ├───dvt-init: package 'dvt'
│   │   ├───dvt-update: package 'update'
│   │   └───test-develop: package 'test-develop'
│   └───x86_64-linux
│       ├───dvt-format: package 'format'
│       ├───dvt-init: package 'dvt'
│       ├───dvt-update: package 'update'
│       └───test-develop: package 'test-develop'
└───templates
    ├───go: template: Go development environment
    ├───node: template: Nodejs development environment
    ├───node14: template: Nodejs (v14) and pnpm (v5) development environment
    └───react-native: template: React Native development environment