Add x-enumNames support

[RicoSuter]

First: Thanks for this awesome project, works like a charm...

Reasons for making this change

Add support for the x-enumNames property which has the same semantics as enumNames but uses the x- prefix to declare it as a custom/extension property. I'm maintaining the NJsonSchema C# library which generates these properties from .NET enums. See https://github.com/RSuter/NJsonSchema/wiki/Enums

I think it makes sense to support this scenario because this prefix is required/defined in the JSON Schema spec. It seems that the proposal for the property enumNames has been rejected in favor of const (see below).

Alternatives

There are also const enums: json-schema-org/json-schema-spec#67 (comment)

Spec for const: json-schema-org/json-schema-spec#58

Checklist

• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style