Parse StandardFormat.Precision in Utf8Format

[neuecc]

Currently Utf8Format can not path StandardFormat.Precision.
https://docs.microsoft.com/en-us/dotnet/api/system.buffers.standardformat.-ctor?view=netcore-3.1#System_Buffers_StandardFormat__ctor_System_Char_System_Byte_

It is unconvinient especially log formatting Cysharp/ZLogger#9

[neuecc]

already it can parse like "{0:D2}". I'll write document.