Add the possibility to specify the placement of sub-menus

[lanceschi]

I'd like to have to possibility to specify the placement of sub-menu, especially since sometimes the automatic placement is not desirable. See the attached screenshot:

The placement of the parent sl-dropdown element is set to bottom-start, and I would like to enforce the same property for the submenus as well.

Can you help me with that?

Thanks in advance

[claviska]

We're open to a PR to add this through the submenu controller, as long as things don't get unwieldy and it continues to flip/shift when the viewport doesn't have sufficient space for the desired placement.

[lanceschi]

Thanks for the prompt reply