Add JavaScript sequence API documentation

[petersalomonsen]

This PR adds comprehensive documentation for the JavaScript sequence API used in the song compiler.

Changes

• Added SEQUENCE_API.md with documentation for:
  • Global functions (setBPM, waitForBeat, playFromHere, loopHere)
  • Track functions (createTrack, TrackerPattern)
  • Note functions (c0-b10 with transpose/fixVelocity methods)
  • MIDI control functions (pitchbend, controlchange)
  • Video/Audio functions
  • Recording functions
  • Song structure functions (definePartStart, definePartEnd)
  • Channel control functions (mute, solo)
  • TrackerPattern class methods (steps, play)
  • Pattern class methods
  • Array extensions (quantize, fixVelocity, repeat)
  • Complete usage example