feat: detect bun package manager

[colinhacks]

Detects usage of bun via npm_config_user_agent and prints the appropriate commands

Successful local run:

$ npm_config_user_agent=bun/1.0 bin/vitepress.js init
┌   Welcome to VitePress! 
│
◇  Where should VitePress initialize the config?
│  ./
│
◇  Site title:
│  My Awesome Project
│
◇  Site description:
│  A VitePress Site
│
◇  Theme:
│  Default Theme
│
◇  Use TypeScript for config and theme files?
│  Yes
│
◇  Add VitePress npm scripts to package.json?
│  Yes
│
└  Done! Now run bun run docs:dev and start writing.

Tips:
- Make sure to add .vitepress/dist and .vitepress/cache to your .gitignore file.