Move to .NET8

- Change TFM and LangVersion
- Better exception throwing (CA1510, CA1512, CA1513)
- Better exception utility method definition (CA1859)
- Prefer comparing `.Count` over calling `.Any()` (CA1860)
- Prefer `.AsSpan()` over `.Substring()` (CA1846)
- Add a few more `scoped`
- Use `RuntimeHelpers.GetUninitializedObject()` instead of `FormatterServices.GetUninitializedObject()`
- Use delegate instead of delegate pointer in variant generic conversions
- Enable EnforceExtendedAnalyzerRules in source generator projects
- Disable CS8981 on structs named movable in Godot.NativeInterop

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/Constants.cs

@@ -1,5 +1,6 @@
 using System.IO;
 using System.Reflection;
+using Microsoft.CodeAnalysis.Testing;
 
 namespace Godot.SourceGenerators.Tests;
 
@@ -7,6 +8,13 @@ public static class Constants
 {
     public static Assembly GodotSharpAssembly => typeof(GodotObject).Assembly;
 
+    // Can't find what needs updating to be able to access ReferenceAssemblies.Net.Net80, so we're making our own one.
+    public static ReferenceAssemblies Net80 => new ReferenceAssemblies(
+        "net8.0",
+        new PackageIdentity("Microsoft.NETCore.App.Ref", "8.0.0"),
+        Path.Combine("ref", "net8.0")
+    );
+
     public static string ExecutingAssemblyPath { get; }
     public static string SourceFolderPath { get; }
     public static string GeneratedSourceFolderPath { get; }