Add watch mode for automatic SVG regeneration

[javierartero]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I need to manually run svgpack every time I add or modify SVG files during development. This breaks the development flow.

Describe the solution you'd like

Add a --watch flag that monitors the SVG directory and automatically regenerates the CSS output on file changes.

svgpack src/assets/svg > output.css --mask --background --watch

The watch mode should:

• Monitor the source directory for SVG changes (add/modify/delete)
• Auto-regenerate output on changes
• Handle errors without stopping

Describe alternatives you've considered

Using external watchers like nodemon or chokidar-cli:

{
  "scripts": {
    "watch:svg": "nodemon --watch src/assets/svg --ext svg --exec npm run build:svg"
  }
}

This works but requires extra dependencies.

Additional context

Would align svgpack with other build tools (Tailwind, TypeScript, PostCSS) that have native watch modes. Could use chokidar for the implementation.