Implement escaped-character handling in Utf8JsonReader and JsonElement (dotnet/corefx#36347)
* Implement escaped-character handling in (Try)GetDateTime(Offset) & (Try)GetGuid in Utf8JsonReader and JsonElement
This addresses https://github.com/dotnet/corefx/issues/36202.
* Address review comments
* Address more comments
* Simplify stackalloc logic
* Update fraction digit cap to 15
* Add (Try)GetGuid to JsonElement
* Address review feedback
* Add FormatException documentation
* Address review feedback
* Address review comments
Commit migrated from https://github.com/dotnet/corefx/commit/
769c5ace33486fecb480c7bf0a572854c9f33375