Return $this in add_field for chaining.

Allows chaining:
```php
$settings_page->get_section( 'general' )
	->add_field( array(
		'slug' => 'footer_text',
		'title' => __( 'Footer Text', 'textdomain' ),
	) )
	->add_field( array(
		'slug' => 'comment_note',
		'title' => __( 'Comment Note', 'textdomain' ),
	) );

```