[FEAT] Add selected functionality for tree component

[xelaint]

💡 Feature request

Feature Name

Add selected UI for tree component

The Desired Behavior

A [daff-tree-item] that is active/selected should have a selected class added to it to properly style it.

Your Use Case

As a designer, I would like to indicate to users if a tree item in a tree is currently selected/active.

Prior Work

Environment

Daffodil version: X.Y.Z
Angular version: X.Y.Z 

 
For Tooling issues:
- Node version: XX  
- Platform:  

Others:

[griest024]

use routerLinkActive

[xelaint]

use routerLinkActive

@griest024 The styles for selected is in the code, but I'm not sure how I can use the routerLinkActive directive in DaffTreeItemDirective so that a selected tree item has the routerLinkActive directive set on it as well.

[griest024]

You can assign this to me