The ornamental design for a lightstring, as shown and described.
FIG. 1 is a first perspective view of a light string showing my new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is an enlarged view of area 9 in FIG. 1.
The broken lines shown in the drawings depict portions of the light string in which the design is embodied that form no part of the claimed design. The dash-dot-dash broken lines encircling the enlarged views in FIGS. 1 and 9 are for annotative purposes only and form no part of the claim. The light string is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.