Migrate ProjectManager to dependency injection

- Created IProjectManager interface
- ProjectManager now implements IProjectManager
- Removed ProjectManager::Get() singleton
- Added IEditorContext::GetProjectManager()
- ProjectPanel now uses m_context->GetProjectManager() instead of singleton
- EditorContext owns ProjectManager instance

editor/src/Managers/ProjectManager.cpp

@@ -9,11 +9,6 @@ namespace fs = std::filesystem;
 namespace XCEngine {
 namespace Editor {
 
-ProjectManager& ProjectManager::Get() {
-    static ProjectManager instance;
-    return instance;
-}
-
 std::vector<AssetItemPtr>& ProjectManager::GetCurrentItems() {
     if (m_path.empty()) {
         static std::vector<AssetItemPtr> empty;