more explicit way of adding modules

[pi0]

Background: some thoughts here.

Nitro modules should be explicitly imported and configured in the user config. Pretty much how a rollup or vite plugin can be configured.

• avoid auto scanning server/modules directory (can be still used by users as convention, but needs explicit import)
• remove support of passing modules as string in config

While this is a breaking change for v3, we never publicly documented modules feature and initial support was also added as an advanced feature. This pattern is also tried on nitro-cloudflare-dev module and proven to work nice.