Olivero: Support more than two levels of menu in primary menu.

Problem/Motivation

In Version 9.2, dropdown menus do not work at all when more than 2 levels of menu selected.

second-level-navigation.js?v=9.2.0:21 Uncaught TypeError: Cannot read property 'classList' of null
    at second-level-navigation.js?v=9.2.0:21
    at NodeList.forEach (<anonymous>)
    at toggleSubNav (second-level-navigation.js?v=9.2.0:19)
    at HTMLLIElement.<anonymous> (second-level-navigation.js?v=9.2.0:62)

Steps to reproduce

Create a main menu of three or more levels. In admin/structure/block/manage/olivero_main_menu set to display 4 menu levels and expand all menu links.

Proposed resolution

Rewrite second-level-navigation.js so that it can cater for more than two levels of menu. This was possible in the javascript for the 9.1 version of Olivero (albeit with some tweaking of the js)