feat: add `SmartOpenDirectory` highlight group

[scottmckendry]

Adds a new highlight group to replace the default Directory group.

The new SmartOpenDirectory group allows users to override the color of the directory in smart-open without changing the main Directory group.

Why?

My other pickers look like this:

With the current highlighting configuration, smart-open looks like this:

With the proposed changes, I can make it look like this instead:

[scottmckendry]

Duplicate of #66 - closing