format: provide formatter inside custom symbol

[Kikobeats]

If you use the formatter with custom symbol, like:

  [customInspectSymbol]() {
    return JSON.stringify(deepObject) // ugh
  }

You are losing formatter capabilities since you have to manually stringify the value by your own.

Ideally, the formatter should be available there:

  [customInspectSymbol]({ format }) {
    return format(deepObject) // yay
  }