Restore support for .NET Standard 2.0

Author: ejball

Directory.Packages.props

@@ -11,6 +11,9 @@
     <PackageVersion Include="xunit" Version="2.9.3" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.5" />
   </ItemGroup>
+  <ItemGroup>
+    <GlobalPackageReference Include="PolySharp" Version="1.16.0" />
+  </ItemGroup>
   <!-- DO NOT EDIT: dotnet-common-props/analyzers convention -->
   <ItemGroup>
     <GlobalPackageReference Include="Faithlife.Analyzers" Version="1.*" />

src/Faithlife.Parsing/ExceptionExtensions.cs

@@ -0,0 +1,17 @@
+#if !NET
+using System.Runtime.CompilerServices;
+
+namespace Faithlife.Parsing;
+
+internal static class ExceptionExtensions
+{
+	extension(ArgumentNullException)
+	{
+		public static void ThrowIfNull(object? argument, [CallerArgumentExpression(nameof(argument))] string? paramName = null)
+		{
+			if (argument is null)
+				throw new ArgumentNullException(paramName);
+		}
+	}
+}
+#endif

src/Faithlife.Parsing/Faithlife.Parsing.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;net8.0</TargetFrameworks>
     <Description>A simple library for constructing parsers in C#.</Description>
     <PackageTags>parser parsers</PackageTags>
     <IsPackable>true</IsPackable>