Bump PKHeX.Core to C#9 language version

Add FrameworkUtil.cs which is compiled if the build isn't net5+, needed to support init setters & records

PKHeX.Core/PKHeX.Core.csproj

@@ -10,7 +10,7 @@
     <PackageIconUrl>https://github.com/kwsch/PKHeX/blob/master/PKHeX.WinForms/icon.ico</PackageIconUrl>
     <PackageProjectUrl />
     <RepositoryUrl>https://github.com/kwsch/PKHeX</RepositoryUrl>
-    <LangVersion>8</LangVersion>
+    <LangVersion>9</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
 

PKHeX.Core/Util/FrameworkUtil.cs

@@ -0,0 +1,21 @@
+#if !NET5
+#pragma warning disable
+// ReSharper disable once UnusedType.Global
+
+namespace System.Runtime.CompilerServices
+{
+    using Diagnostics;
+    using Diagnostics.CodeAnalysis;
+
+    /// <summary>
+    ///     Reserved to be used by the compiler for tracking metadata.
+    ///     This class should not be used by developers in source code.
+    /// </summary>
+    [ExcludeFromCodeCoverage, DebuggerNonUserCode]
+#endif
+    internal static class IsExternalInit
+    {
+    }
+}
+
+#pragma warning restore

PKHeX.Drawing/Properties/Resources.Designer.cs

@@ -8,6 +8,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+[assembly: System.Resources.NeutralResourcesLanguage("en")]
 namespace PKHeX.Drawing.Properties {
     using System;
     

PKHeX.WinForms/Properties/Resources.Designer.cs

@@ -8,6 +8,7 @@
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+[assembly: System.Resources.NeutralResourcesLanguage("en")]
 namespace PKHeX.WinForms.Properties {
     using System;