Add isSingletonCasesEnum flag

[sake92]

It is useful to disambiguate between "singleton cases"/simple enums and ADTs.

My concrete use case was JSON serialization:

• simple enums get written as simple strings
• ADTs get written as objects with @type field, and other fields if they have them

Implementation-wise, the official docs say that only "singleton cases" enums will have a values method.

[adamw]

Thanks - this isn't binary-compatible, but there's a scala3-v2 branch that can accept these changes :)

[sake92]

Thanks @adamw !
Do those artifacts get published too?

[adamw]

Not yet - but we'll get there eventually :)

[sake92]

Could this be merged now?

[adamw]

@sake92 see #552