semi-auto derivation: Automatically derive instances for nested classes (at least for Scala 3)

[jatcwang]

i.e. similar behaviour to https://github.com/plokhotnyuk/jsoniter-scala?tab=readme-ov-file#how-to-use

feedback based on: https://github.com/jatcwang/difflicious/issues/29#issuecomment-3027548056