Add anchor/alias support in JsonEventEmitter

[am11]

Today, serializing the following YAML with .JsonCompatible() throws Unhandled exception. System.NotSupportedException: Aliases are not supported in JSON:

x: &anchor1
  z:
    v: 1
y:
  k: *anchor1

This PR adds support for anchors and aliases, such that it produces JSON replicating the anchored value:

{
   "x": {
      "z": {
         "v": "1"
      }
   },
   "y": {
      "k": {
         "z": {
            "v": "1"
         }
      }
   }
}

It makes it very close to other frameworks, e.g. https://www.convertjson.com/yaml-to-json.htm (modulo quoted numeric value "1" in our case when deserializing-then-reserilizing a dynamic object vs. unquoted 1 in theirs).

Also:

• fix a warning Serialization\SerializerBuilder.cs(557,64): warning CS8625: Cannot convert null literal to non-nullable reference type.
• rename YamlDotNet/Serialization/YamlMember.cs to match class name: YamlDotNet/Serialization/YamlMemberAttribute.cs.
• add missing copyright header in YamlDotNet/Core/EmitterSettings.cs.