node 14: Object.prototype.toString doesn't use constructor name

[romgrk]

const attr = new Pango.AttrSize(10)
console.log(attr.toString())
// in node 12: [object AttrSize]
// in node 14: [object Object]

Not sure what is the cause of this change. We could preserve the old behavior by adding this to all our boxeds:

AttrSize.prototype[Symbol.toStringTag] = 'AttrSize'