Add TFM netstandard2.0 (#150)

Author: ElderJames

src/Markdown.ColorCode/Markdown.ColorCode.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-  <TargetFrameworks>net6.0;net7.0;net8.0</TargetFrameworks>
+  <TargetFrameworks>netstandard2.0;net6.0;net7.0;net8.0</TargetFrameworks>
+  <LangVersion>latest</LangVersion>
   <ImplicitUsings>enable</ImplicitUsings>
   <Nullable>enable</Nullable>
 	<TreatWarningsAsErrors>true</TreatWarningsAsErrors>