`fm_added_element` event is triggered on proto element when Add More position is set to 'top'

## Expected Behavior

`fm_added_element` should fire on the newly-added element, making that available as `Event.target`.

## Actual Behavior

When I set `'add_more_position' => 'top'`, the `fm_added_element` event fires on the prototype element instead of the newly-added element.

This works as expected when using the default `'add_more_position' => 'bottom'`.

## Steps to Reproduce

Create a new `Fieldmanager_Group` with some children and set `'add_more_position' => 'top'`.

Load the following script somewhere:

```js
jQuery(document).ready(function($) {
$(document).on('fm_added_element', function(e) {
console.log(e.target);
});
});
```

Go to wherever you are displaying the group and add a new item.

Open your browser's developer tools console and note that the `fmjs-proto` class is present in the element's `classList`.

## Possible Solution

This is the problematic code:

https://github.com/alleyinteractive/wordpress-fieldmanager/blob/5213e079c9a5f5d11d78f6fc0dc1608af50d3743/js/fieldmanager.js#L161-L165

Specifically:

https://github.com/alleyinteractive/wordpress-fieldmanager/blob/5213e079c9a5f5d11d78f6fc0dc1608af50d3743/js/fieldmanager.js#L165

When `add_more_position == 'top'`, `.first()` should be used to find the new element instead of `.last()`.