Add option to create a banner css themes with a filter

[walbo]

Makes it possible to add new banner themes without touching a line of CSS.

Ex. on adding a pink theme

function themes( array $themes ) : array {
	$themes['pink'] = [
		'label'           => 'Pink',
		'backgroundColor' => '#f0a',
		'color'           => '#000',
	];

	return $themes;
}
add_filter( 'hogan/module/banner/themes', __NAMESPACE__ . '\themes' );

Also updated the frontend build system to use Heisenberg