* SUMMARY
Expose a way for developer provided object previews.

* IDEA (nvasilyev)

The idea here is that instead of just showing the first 5 properties of an object in Object Previews, we could show more useful previews based on what the developer of the object may deem most useful to expose.

For example an object might display poorly:

  js> obj
  ▶︎ MyObject {_root: Object, __owner: undefined, __hash: undefined...}

We can provide a hook for the developers to provide better previews. For example, we could check if the object has a Symbol.preview getter in the prototype chain, and if so use that object in the prototype chain

For example:

  MyObject.prototype = {
    toJSON() { ... },
    get [Symbol.preview]() { return this.toJSON(); },
  };

Would produce:

  js> obj
  ▶︎ MyObject {x: 2, y: 42, color: "red"}


* NOTES
- "Symbol.preview" is probably bad, since that is a built-in object.
- "console.preview" is likely better, after all this is for the "console" / debugger