Refactor reused enum-based macros

[kciesielski]

Resolves #3206
This part moves isScalaEnum[T] from json-pickler to core internals for potential reuse. It also extracts the part that checks for object/enum children.
By the way, I noticed that the implementation I had in json-pickler wasn't recursive like the implementation used in ValidationMacros. Now it's the common one used, with a parameter that tells whether to fail if an unexpected child flag is found.

I could not think of any example to represent a nested case to test the actual recursion. Any ideas? :)

[adamw]

This name can be confusing, because this checks for more than a simple scala enum.

Maybe we could assume the following convention, using the name "enum" for all cases of Scala 3 enums (with or without parameters). On the other hand "enumeration" would mean "true" enumerations, that is parameterless enums or sealed traits with only case objects as children. It's not perfect as one is an abbrievation of the other, but I think if used consistently (we would probably need to document it in some developer docs - maybe readme?) it would be clear what we mean.

[kciesielski]

Makes sense, I propose 'contributing.md' for such documentation. It also looks like a good place for versioning scheme #3111, wdyt?
Actually, versioning scheme should be in stability.md, it's something that may be interesting to users, not only contributors.