Fix #2548: TreeNode typescript add expanded attribute

[melloware]

###Defect Fixes
Fix #2548: TreeNode typescript add expanded attribute

I am writing my own custom tree node filtering algorithm and I based it on the current file name based filtering algorithm in the tree component.

I noticed this code:

            if (matched) {
                node.expanded = true;
                return true;
            }

But in TypeScript my code says that expanded is not a valid property of TreeNode.