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