Rename Impl classes to follow Unity naming convention

- SelectionManagerImpl -> SelectionManager
- EditorContextImpl -> EditorContext
- Removed unused SceneManagerImpl and ISceneManager

The Impl suffix was inconsistent with Unity naming conventions.

editor/src/Layers/EditorLayer.cpp

@@ -7,7 +7,7 @@
 #include "panels/ConsolePanel.h"
 #include "panels/ProjectPanel.h"
 #include "Core/IEditorContext.h"
-#include "Core/EditorContextImpl.h"
+#include "Core/EditorContext.h"
 #include <imgui.h>
 #include <imgui_internal.h>
 
@@ -22,7 +22,7 @@ void EditorLayer::SetContext(std::shared_ptr<IEditorContext> context) {
 
 void EditorLayer::onAttach() {
     if (!m_context) {
-        m_context = std::make_shared<EditorContextImpl>();
+        m_context = std::make_shared<EditorContext>();
     }
     
     m_menuBar = std::make_unique<MenuBar>();