restructure repo into mono repo with melon

[mortenboye]

Restructure FlutterMidiCommand into a Melos-based federated plugin workspace

• split the single-package plugin into a workspace with platform interface, Android, Darwin, Linux, Web, Windows, and BLE packages
• refresh the core API with new transport and MIDI message parsing support, plus platform-specific fixes like Windows disconnect/hot-plug handling and improved Web multi-port pairing
• update the example app, smoke tests, and package docs to match the new workspace layout
• add broader CI coverage and a publish workflow for the federated packages
• remove legacy root plugin scaffolding and regenerate API/docs artifacts for the new structure