Add `count` prop to OuiTreeView

[KrooshalUX]

• [ ] Count prop should reflect the number of OuiTreeNode children, and should be optional to include

• [ ] Reference count styling from OuiFacet and OuiFilterGroup to build styling while retaining hover styling of OuiTreeView - spacing between text & count = $ouiSizeS Examples: - Default:

- Disabled:

Discuss: What do we think about using the styling of selected that is present in OuiFacet and OuiFilterGroup for the isExpanded state of OuiTreeGroup ? Do we think that the expansion itself is visual communication or not? IMO, the expansion itself is sufficient, but for the sake of complete exploration here are examples of count changing color when a user is interacting with an element that contains a count: