Eliminate single-property schemas.

Description:

Supporting both single and multi prop schemas have made it so most A-Frame internal-related codepaths (and stuff like Inspector, aframe-react) need to branch and check to support both, often causing bugs.

Also causes API breakages when a component decides to go from single to multi prop.

We can eliminate single-prop schemas.

To support components such as visible or perhaps position, we can perhaps have components specify a designated "default property". Where if only a plain value is passed into setAttribute (e.g., true), it will set the designated property.

registerComponent('visible', { 
  schema: {
    visible: {default: true, primary: true}
  }
});

// setAttribute('visible', true);
// equivalent to setAttribute('visible', 'visible', true);

registerComponent('position', { 
  schema: {
    position: {type: 'vec3', primary: true}
  }
});
// setAttribute('position', '0 0 1');
// equivalent to setAttribute('visible', 'position', '0 0 1');
// or we can split to X / Y / Z...

Start by tracing the registerComponent schema handling and the setAttribute paths described in the issue, including consumers such as Inspector and aframe-react. Determine how single- and multi-property schemas currently branch, then define the intended primary-property behavior and compatibility requirements before implementation. Done means the schema model no longer needs separate single-property handling while the proposed plain-value usage remains supported.