Refactor project structure and enhance functionality

Changed the project namespace from `Ghost.Editor` to `Ghost.App` across multiple files.
Changed the `InternalsVisibleTo` attribute in `AssemblyInfo.cs` to include `Ghost.App`.
Changed the `ProjectRepository` class to add new asynchronous methods for retrieving projects by ID, name, and metadata path.
Changed the `ProjectService` class to utilize the new asynchronous project loading methods.
Changed the `SceneGraph` classes to improve node management and serialization.
Changed the `EntityManager` class to enhance entity management with new component handling methods.
Added new test classes, `EntityTest` and `SerializationTest`, to ensure reliability in entity and serialization systems.
Added the `Ghost.App` project file to establish a modular project structure.
Added the `Ghost.Generator` project for automated component serialization code generation.
Updated UI components to reflect the new namespace for proper functionality.

Ghost.Editor/Models/ExplorerItem.cs

@@ -1,27 +0,0 @@
-using System.Collections.ObjectModel;
-
-namespace Ghost.Editor.Models;
-
-internal class ExplorerItem(string name, string path, bool isDirectory)
-{
-    public string Name
-    {
-        get;
-    } = name;
-
-    public string Path
-    {
-        get;
-    } = path;
-
-    public bool IsDirectory
-    {
-        get;
-    } = isDirectory;
-
-    public ObservableCollection<ExplorerItem>? Children
-    {
-        get;
-        set;
-    }
-}