Handlebars does not support class instances in the template data

When rendering a compiled template with an array of custom class instances, [this line](https://github.com/handlebars-lang/handlebars.js/blob/f422bfdf3ed321c32c6eecfaa778de46275fb76d/lib/handlebars/runtime.js#L70) butchers the input class instance replacing it with a plain object with all custom class behaviour (getters, setters, methods etc) removed. This breaks my app.

Please remove all destructive use of `Util.extends()` on the input data - leave the input data as it is.

Some pseudocode to illustrate my case. Please let me know if you need anything else.

```js
class Car {
wheels = []
maxSpeed
colour

constructor (data) {
this.maxSpeed = data.maxSpeed
this.colour = data.colour
}

hasFourWheels () {}
}

const cars = [
new Car({ maxSpeed: 40, colour: 'silver' }),
new Car({ maxSpeed: 30, colour: 'white' })
]

handlebars.registerHelper({
customHelper: function () {
/* Fails as the input data is no longer a `Car` instance */
if (this.hasFourWheels()) {
// do something
}
}
})

/* myTemplate includes use of the `customHelper` helper. */
const compiled = handlebars.compile(myTemplate)
/* throws an error */
compiled(cars)
```

Start in lib/handlebars/runtime.js at the linked line around line 70 and trace how template data is handled during rendering. Confirm that class instances retain their custom behavior, then verify the compiled-template example no longer loses methods, getters, or setters.

Written by the indexing model from the issue text.