feat: add isDisposed method to components

[misteroneill]

Description

Usually, when we need to check if a component (or player) has been disposed, we check if its element exists:

if (player.el()) {
  // Do something...
}

It would be clearer if we had a method for that:

if (!player.isDisposed()) {
  // Do something...
}

Specific Changes proposed

• Add isDisposed method to the base component.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
• Reviewed by Two Core Contributors