v0.0.1 of sound_dart

Sound for Dart and Flutter with the goal of no additional system dependencies. Dart (no Flutter dependency) with native audio from Rust via FFI. Changelog excerpt: - Initial scaffolding. - Core API: Sound, SoundSource, Playback, pluggable SoundBackends. - FFI backend that plays WAV via the sound_cliRust library; Linux playback over ALSA (PipeWire). Web-safe via conditional imports. - WebAudio backend for the browser (selected automatically on web). - Native decoding of WAV/MP3/OGG-Vorbis/FLAC via the pure-Rust symphonia(no system dependencies); the browser decodes natively on web. - Per-voice volume (setVolume, volume:) and looping (`setLoo[...]