[feat] sfpegNavigationMenuCmp - Allow sub-menus in vertical list configuration

[brave-hawk]

Is your feature request related to a problem? Please describe.
Request to allow sub-menu items when the sfpegNavigationMenu component is used as a site-map.
In the experience builder, we may add Menu Items of type: 'Menu Label' with sub-menus.
However, these sub-menus do not appear when used a sitemap

Describe the solution you'd like
It would be nice to have the sub-menu items configured appear as children, in the vertical list configuration like so:

Describe alternatives you've considered
No alternatives considered at the moment.

[brave-hawk]

Pull request here: #5

Would be glad to have this reviewed

[pegros]

Thanks for your contribution @brave-hawk.
Implemented it slightly differently and included also the list display mode.
FYI @NicolasGuigui