feat: add Launcher Next .NET 8 foundation

Launcher.Next.Core/PackageManifestJson.cs

@@ -0,0 +1,7 @@
+using System.Text.Json;
+namespace Launcher.Next.Core;
+public static class PackageManifestJson
+{
+    private static readonly JsonSerializerOptions Options = new() { PropertyNameCaseInsensitive = true };
+    public static PackageManifest Parse(string json) => JsonSerializer.Deserialize<PackageManifest>(json, Options) ?? throw new InvalidOperationException("MANIFEST_EMPTY");
+}