fix(dual-list-selector): tree to support focus-within, hc border on hover/focus - #7829

fixes #7795

• Adds HC border on hover/focus for tree variant
• Adds background color change when focusing on something (either the expand toggle or checkbox) in a tree item - this is a net new style to the default styles, not just HC - @lboehling is that ok? This matches tree view styles.

Previously looks like this:

Now looks like this:

For comparison this is tree view, which has the same styling: