The ornamental design for a lunchbag, as shown and described.
FIG. 1 is a perspective view of a lunch bag showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines throughout the drawings represent stitching and form no part of the claimed design. The remainder of the broken lines depict portions of the lunch bag that form no part of the claimed design.