Rename JsonValueType to JsonValueKind and JsonElement.Type to JsonElement.ValueKind. (dotnet/corefx#38943)
* Add JsonDocument.Write, JsonProperty.Write, and rename JsonValueTupe to
JsonValueKind.
* Add JsonDocument.Write API tests.
* Don't use GetPropertyRawBytes and add JsonProperty.Write API tests.
* Make GetPropertyRawValue private again.
* Remove the new Write APIs on JsonDocument, JsonProperty.
Commit migrated from https://github.com/dotnet/corefx/commit/
f08256cbba2b249cc1e5ca67cf4a1c074adf035b