Tree node collapsed state

[skoro]

This PR lets you control the node collapsed state as well as the default tree collapsed state. For example, for long trees it's better to have them initially collapsed to overview all the root nodes and manually expand some nodes.

By default, collapsed is false but overriding resource's getNodeCollapsedState() method, user can set a whole tree as collapsed or selectively collapse nodes:

    public function getNodeCollapsedState(?Model $record = null): bool
    {
        // All the tree nodes will be collapsed.
        // return true;

        // Collapsed will be a node with id 3.
        // return $record->id == 3;

       // The tree nodes will be expanded.
       return false;
    }