While internally enums are mostly the same as classes, their output in
`ReflectionClass::__toString()` should show the enum as the developer wrote it,
rather than as the engine stored it. Accordingly
- Say that the enum is an enum, not a final class
- Include the backing type, if any, in the declaration line
- List enum cases separately from constants, and show the underlying values, if
any
GH-15766
