[new-package] Content collection injection

[Fryuni]

Double trouble:

• Inject in the file system
• Inject virtually

Thanks to @BryceRussell for the ideas

[Fryuni]

// Without any extra config
export const idea7 = {
  docs: defineCollection({

  }),
}

// With some config to disable the auto-injection
export const idea7a = {
  darculaColors: darculaColorCollection({

  }),
  docs: defineCollection({

  }),
}