The ornamental design for a treelightstring, as shown and described.
FIG. 1 is a front view of a tree light string showing my new design;
FIG. 2 is a rear view of the tree light string;
FIG. 3 is a side view of the tree light string;
FIG. 4 is a top view of the tree light string;
FIG. 5 is a bottom view of the tree light string;
FIG. 6 is a rear view of the tree light string; and,
FIG. 7 is a top front side perspective view of the tree light string.
The tree light string is shown with symbolic breaks in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.