Feature: Add native MP3 decoding support via minimp3

[MasterAlexS]

Description

This PR adds native support for decoding and playing MP3 files in the C++ backend, matching the existing architecture used for WAV and OGG formats.

To keep the implementation lightweight and avoid heavy external dependencies, this uses the minimp3 header-only library (which is already included in the ecosystem for the HashLink target).

Changes made

• Submodule: Added minimp3 as a Git submodule in lib/minimp3.
• Build System: Updated Build.xml to include the LIME_MP3 flag and created lib/mp3-files.xml for the build toolkit configuration.
• C++ Decoder: Implemented include/media/containers/MP3.h and src/media/containers/MP3.cpp with the Decode method. It handles both memory buffers and file paths. Memory is safely mapped to Haxe's ArrayBufferView using alloc_null() with explicit byte assignment.
• CFFI Integration: Updated all four loading functions in src/ExternalInterface.cpp (lime_audio_load_bytes, hl_audio_load_bytes, lime_audio_load_file, hl_audio_load_file) to properly route MP3 checks to the new decoder.

Testing

• Successfully compiled on Linux (CachyOS).
• Tested playback using a sample application calling AudioBuffer.loadFromFile() and successfully played the decoded MP3 data through AudioSource.
• LimeMP3Testing.zip