Support SmartEnums as dictionary keys

[Steve-OH]

This addresses #242 by adding support for SmartEnums as dictionary keys when using System.Text.Json. It adds ReadAsPropertyName and WriteAsPropertyName implementations to both SmartEnumNameConverter and SmartEnumValueConverter.

Note that there isn't a good way to use the JsonConverter attribute when you want to use a dictionary with a SmartEnum as a key. Instead, you have to explicitly add the appropriate SmartEnum converter to the serializer options (see the tests for examples of this). While it would be possible to create a somewhat more generic dictionary converter that would allow you to use the JsonConverter attribute, there would have to be separate converter implementations for each of the different dictionary types (IDictionary, IImmutableDictionary, IList<KeyValuePair<TKey, TValue>>, etc.), which doesn't seem worth the effort.

A note on the value converter tests: There doesn't seem to be a way to create a C# anonymous object where the property name is not a valid identifier (e.g., you can't create an anonymous object that serializes to { "1.5": "foo" }, so I had to jump through a couple of small hoops to create the reference JSON for the (de)serialization tests.