refactor: move inspector to separate package

[bluwy]

Closes https://github.com/sveltejs/vite-plugin-svelte/issues/558

• Created packages/vite-plugin-svelte-inspector with JSDoc
• Named it @sveltejs/vite-plugin-svelte-inspector (quite a long name)
• Copied most of the code, except some small refactors including:
  • Debug logs are done directly with the debug package
  • Warning and error logs uses Vite's config.logger API
  • Removed some logs which I'm not sure was helpful (happy to walk back if needed)
  • Refactor idToFile to not handle /@fs/, since that should never happen