Improved performance with strings

All multiple string concatenations using "+" changed to string.Concat.
RegexConverter.WriteBson, JsonSerializerInternalBase.IsErrorHandled, JsonSerializerInternalWriter.CheckForCircularReference  and JsonPosition.FormatMessage implementations changed to use StringBuilder.
JToken.Path property implementation improved by using AppendFormat instead of 3 string concatenations.
MemoryTraceWriter.Trace improved Format method usage.
Added some extension methods to work with StringBuilder: Trim and EndsWith. Code was taken from my project https://github.com/xperiandri/StringBuilderExtensions