feat: add experimental directory and trailing slash handler

[userquin]

This PR also includes:

• add logical manifest entries instead replacing original ones
• don't add manifestTransform when provided
• included test folder in pwa-simple example, with index.astro and about.astro routes: /test, /test/, /test/about and /test/about/ working
• map 404.astro to 404, Astro won't generate the 404/index.html and the sw will fail when precaching 404.html page
• include StackBlitz links to run the examples: added to readme and included hints to run the PWA
• bump to pnpm 8.9.2

supersedes #26
closes #23