feat: add sub-menu(s) to CellMenu & ContextMenu plugins

[ghiscoding]

• also merge command/option code since we had a lot of duplicate code to do roughly the same thing but with different list (command/options)
• currently only works by click event on sub-menus, I tried to implement it with mouseover/mouseout but it's a lot more complex (we need to know if we are mousing over the correct sub-menu or over something else and this and that... wow just too much work)... so click should be enough for now, even though it's slightly less user friendly
• this is the 1st PR but in theory it should be easy enough to replicate for other menu plugins (GridMenu, HeaderMenu)

TODOS

• add sub-menus to Cell Menu plugin
  • add Cypress E2E tests
• add sub-menus to Context Menu plugin
  • add Cypress E2E tests
• add slick-submenu class when it's a sub-menu
• add sub-menu level number for optional custom styling
  • i.e. Grouping does it in 2 ways: 1-add slick-group-level-2 & 2-add level="2" attribute
• verify that sub-menu(s) is always at an expected position, it seems incorrect for some sub-menus
• add subMenuTitle & subMenuTitleCssClass to optionally add sub-menu title & custom styling
• opening a sub-menu then open a sub-menu from another tree doesn't close previous sub-menu that becomes orphan