Add initialization command

[yyx990803]

• Add a new command: vitepress init
  • Users can invoke this after installing vitepress with npx vitepress init
• Ask a few CLI questions:
  • Root of your markdown files (default: cwd)
  • Site title / description
  • Use the default theme / default theme + customization / complete custom theme?
  • Inject npm scripts? (should be based on root)
• Scaffold the project in desired root
  • .vitepress/config.js with title, description, and a comment with link to app config docs
  • example.md with examples of built-in markdown features
  • If default theme:
    • index.md w/ example home page frontmatter
    • Example nav / sidebar config with links to example.md and VitePress docs
    • If default theme + customization: add .vitepress/theme/index.js and .vitepress/theme/style.css with theme slot and CSS var customization exmaples
  • If custom theme:
    • add .vitepress/theme/index.s + .vitepress/theme/Layout.vue for a simple custom theme example