chore: Also allow Net8

Author: furesoft

Source/Directory.Build.props

@@ -8,7 +8,7 @@
     <RepositoryUrl>https://github.com/furesoft/Silverfly</RepositoryUrl>
     <Description>A little framework to build parsers</Description>
     <PackageTags>parser, pratt</PackageTags>
-    <TargetFramework>net9.0</TargetFramework>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <AllowedOutputExtensionsInPackageBuildOutputFolder>$(AllowedOutputExtensionsInPackageBuildOutputFolder);.pdb</AllowedOutputExtensionsInPackageBuildOutputFolder>
   </PropertyGroup>