Changed all Plugin classes and PluginManager to be sealed, preventing further inheritance. This improves code safety and clarifies the intended usage of these classes.
This commit updates various NuGet package references to newer versions across multiple QuickLook plugin and main project .csproj files. These updates include dependencies such as WixToolset.Dtf.WindowsInstaller, UTF.Unknown, FreeTypeSharp, Microsoft.Web.WebView2, Magick.NET-Q8-AnyCPU, Google.Protobuf, MsgReader, HelixToolkit, bblanchon.PDFiumV8.Win32, Melanchall.DryWetMidi, WPF-UI.Violeta, and Lib.Harmony, ensuring the projects use the latest compatible releases.