The ornamental design for a handheld lighter, as shown and described.
FIG. 1 is a front perspective view of the handheld lighter of the present design;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof.
Broken lines illustrate portions of the article that form no part of the claimed design. The article is shown with a symbolic break in its length, represented by dash-dot break lines. The appearance of any portion of the article between the dash-dot break lines forms no part of the claimed design.