Move Enum-related `SerializationFeature`s into `EnumFeature` (3.0)

[cowtowncoder]

(similar to #5079 )

There are some SerializationFeatures that would make more sense as more specific EnumFeatures. Let's make them. This includes:

• WRITE_ENUMS_USING_TO_STRING (default: true; was false in 2.x)
• WRITE_ENUMS_USING_INDEX (default: false)
• WRITE_ENUM_KEYS_USING_INDEX (default: false)

[JooHyukKim]

Will get to this and the other one 👍🏼