Support expand button in <wa-tree-item> even if it has no children

[mblinsitu]

Tree items only show an expand button if they have children. This makes sense in many use cases, but is inconvenient when displaying directory trees, because empty directories look like files. More generally, there are trees where it is important to distinguish nodes from leaves (and while I can use icons to distinguish them, it is inconsistent to have some nodes with an expand buttons and others without).

The macOS Finder, for example, displays empty folders with an expand button, which also makes it possible to open them. This is useful, e.g. when viewing a shared folder, so the user can decide whether or not they want to see the content as it changes over time.

Currently, WaTreeItem has an isLeaf property that is set to !this.lazy && this.getChildrenItems().length === 0.

One possibility would be a new (optional) attribute, e.g. expandable, that overrides this default behavior. As far as I can tell, that's the only change that's needed.