Feat: Enum::StringConverter(T)

[caspiano]

Adds Enum::StringConverter(T), a converter to be used with JSON::Serializable to write and read
an Enum exclusively as a String.

This is useful if you wish to serialise to String, instead of
the default of a number representation of the Enum.

Closes #9881

Includes fixes to minor typos in a few serialization examples.

Usage:

require "json"
                                                                            
class EnumStringMessage
  include JSON::Serializable
                                                                            
  enum Hint
    Up
    Down
  end
                                                                            
  @[JSON::Field(converter: Enum::StringConverter(EnumStringMessage::Hint))]
  getter hint : Hint
end
                                                                            
message = EnumStringMessage.from_json(%({"hint":"Up"}))
message.hint    # => EnumStringMessage::Hint::Up
message.to_json # => %({"hint":"Up"})

[Sija]

My 2c:

• I'd make it a default
• I'd use to_s.underscore for serializing

[caspiano]

@Sija Yeah, I agree, I'd much prefer to_s.underscore.

I'll whip up another PR.