[Sound] Add speed accessor

[beefchimi]

We should be able to adjust the playbackRate of each Sound. This probably should be restricted to Sound and unavailable on Stack or Earwurm.

Changing playbackRate also affects "pitch" - the faster the audio is played, the higher pitch is sounds. At the moment, there is no native way to "preserve pitch", but it is in discussion here: WebAudio/web-audio-api#2487

We should try to implement a custom solution (if possible) to preservePitch.

Expectation

const sound1 = await stack.prepare();
// Should be `1` by default.
const currentSpeed = sound1.speed;

// sound1.speed = 2;
sound1.setSpeed(2, 1000);

const sound2 = await stack.prepare({speed: 2});
const sound3 = new Sound(...props, {speed: 0.5, preservePitch: true});

Key points

• Defaults to 1.
• Can “ramp to” changed values.
  • Reference: [Earwurm] Accessor vs method pattern #29
• Consider applying a custom default in the constructor?