Add initial PlatformIO configuration and main application code

- Created platformio.ini for ESP32 Devkit configuration with DFPlayer Mini library dependency.
- Implemented main application logic in main.cpp, including LED and buzzer control, and MP3 playback functionality using DFMiniMp3 library.
- Added notification class for handling MP3 player events and errors.
- Set up input pins for buttons and output pins for LEDs, with synchronization logic.
- Included basic setup and loop functions to manage state and playback.
- Added README for test directory to outline unit testing purpose and resources.

platformio.ini

@@ -0,0 +1,17 @@
+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[env:esp32doit-devkit-v1]
+platform = espressif32
+board = esp32doit-devkit-v1
+framework = arduino
+monitor_speed = 115200
+lib_deps = 
+	makuna/DFPlayer Mini Mp3 by Makuna@^1.2.3