[ic-tree-view] add action button slot to heading area

## Summary
This was left out of the initial implementation to get a first iteration out quicker, but it would still be useful to have a slot in the heading area of tree view for icon buttons/actions.

## 💬 Description
An action slot on the right hand side of the heading area so that users could slot in an interaction button like in cards.

## 💰 Use value
Gives tree view more flexibility for users.

Start by locating the ic-tree-view component and reviewing how cards expose action buttons or slots. Confirm the heading-area markup and existing component tests, then add coverage showing that slotted content renders on the right side of the heading; done means the action slot works without changing existing tree-view behavior.

Written by the indexing model from the issue text.