Start cross-compiling pureconfig-core to Scala 3.0.0-M2

[jcazevedo]

Depends on #890.

This PR starts cross-compiling pureconfig-core to Scala 3.0.0-M2.

This makes the Derivation macro a no-op in Scala 3. I'm not sure we should implement it with Scala 3 macros, since errors in implicit chains are much better now, pointing the user to the most specific implicit argument that was missing in the implicit search. I'm actually inclined to drop it entirely, since the Derivation type leaks into PureConfig's external API, the macro is hard to maintain and we can recommend users in Scala 2 to use splain for better implicit not found errors. @leifwickland and @ruippeixotog: If you agree with this, I'll open a separate PR to remove the Derivation type and update the documentation to provide a guide to using splain for debugging implicit not found errors.

My plan is to follow-up with the necessary updates to support generic derivation of ConfigReader and ConfigWriter instances in another PR, hopefully still during this week. I think we should be able to support this directly in pureconfig-core using the type class derivation features of Scala 3 (and taking advantage of the derives machinery). We probably still want to keep pureconfig-generic for compatibility purposes and to provide the configuration mechanisms for derivation we currently support with hints.

[ruippeixotog]

Thanks for doing this, I'm excited to start supporting Scala 3.0! Apart from one comment regarding failWithType the code changes look good to me, but I think we need to simplify the build.

[ruippeixotog]

Regarding Derivation I'm up for deprecating it 👍 I'm happy to know better implicit explanation is being introduced in Scala 3 and splain is becoming a mature option, as this shouldn't be left to libraries like PureConfig anyway and the current approach is rather hacky.

[ruippeixotog]

Thanks @jcazevedo!