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.

test/README

@@ -0,0 +1,11 @@
+
+This directory is intended for PlatformIO Test Runner and project tests.
+
+Unit Testing is a software testing method by which individual units of
+source code, sets of one or more MCU program modules together with associated
+control data, usage procedures, and operating procedures, are tested to
+determine whether they are fit for use. Unit testing finds problems early
+in the development cycle.
+
+More information about PlatformIO Unit Testing:
+- https://docs.platformio.org/en/latest/advanced/unit-testing/index.html