How to select multiple items in treeview VS Code Extension

I have followed this guide for creating a tree view that loads a directory so far. Now I want to add checkboxes with each tree item. How can I achieve this? If not checkboxes, how can I make it so that one can select multiple items of tree nodes?

I found a way, that is, using TreeViewOptions<T> but I don't know how to use this exactly. This requires treeDataProvider. How can I assign a TreeDataProvider to this?