This is the Class Menu which is used by Tigra.
Install the package using composer.
composer require tigra/menu
or by editing composer.json yourself and adding:
"require": {
"tigra/menu": "*"
}
This package includes the CSS and JavaScript file from Github repository Bootstrap 4 Responsive Navbar with Multi level Dropdowns from developer BootstrapThemes.co.
If you want to use multi level Dropdowns for your Bootstrap menu, don't forget to add following lines to your twig-template:
<script src="{{ BASE_URL }}/vendor/tigra/menu/src/js/bootstrap-4-navbar.js"></script>
<link href="{{ BASE_URL }}/vendor/tigra/menu/src/css/bootstrap-4-navbar.css" rel="stylesheet">