Add post-root-package-install script for generating Salts/Keys

Adds a script that's run on the `post-root-package-install` event. The script copies `.env.example` to `.env` and generates all the necessary salts/keys for WP and appends them to `.env`.

In interactive mode, there's a prompt for the generation. In non-interactive mode (`-n` flag), the `config.generate-salts` option is read from `composer.json` to determine if it's run or not.