[ic-tree-view] add checkbox to tree items

## Summary
In the designs, there's a slot for adding a checkbox to tree items. This was left out initially to get a first iteration of tree view done quicker.

## 💬 Description
A checkbox on the right hand side of tree items.

Some considerations:
Does focusing the tree item focus the checkbox instead when there's one present?
If a parent tree item has a checkbox, do we want a prop to let the developer decide if we want that to be passed down to child tree items so they all have checkboxes too?
Will it be a slot that the user can control more or do we want it to be a prop so we have more control over the implementation? e.g. so someone doesn't add a status tag in the checkbox slot.

## 💰 Use value
Gives a different use case to tree view e.g. selecting multiple files at once.

## ✏ Designs

Start with the tree-view implementation and the linked design image, then resolve the open questions about focus behavior, parent-to-child propagation, and whether this is a slot or prop. Done means the agreed checkbox behavior is implemented for tree items and matches the design.

Written by the indexing model from the issue text.