The ornamental design for a lunchbag, as shown and described.
FIG. 1 is a front perspective view of the lunch bag showing my new design.
FIG. 2 is a front view of the lunch bag.
FIG. 3 is a rear view of the lunch bag.
FIG. 4 is a left side view of the lunch bag.
FIG. 5 is a right side view of the lunch bag.
FIG. 6 is a top view of the lunch bag; and,
FIG. 7 is a bottom view of the lunch bag.
Any shading and cross-hatching are not features of the design but are utilized to illustrate the surface contours of the lunch bag design in the drawings.