chore(application): refactor app lifecycle and add entity spawner module

- updated CMakeLists.txt to include new module targets
- refactored application.hpp + application.cpp
- added new `entity_spawner` headers + sources:
  - entity_spawner.hpp
  - entity_spawner.cpp

CMakeLists.txt

@@ -486,6 +486,7 @@ endif()
 set(WOWEE_SOURCES
     # Core
     src/core/application.cpp
+    src/core/entity_spawner.cpp
     src/core/window.cpp
     src/core/input.cpp
     src/core/logger.cpp