The ornamental design for a zester, as shown and described.
FIG. 1 is a front perspective view of a zester in a covered state, showing our new design;
FIG. 2 is a front perspective view thereof in an uncovered state;
FIG. 3 is a rear view thereof in a covered state;
FIG. 4 is a rear view thereof in an uncovered state;
FIG. 5 is a left side view thereof in an uncovered state; and,
FIG. 6 is a left side view thereof in a covered state.
Right side views of the zester in the uncovered and covered states are omitted as they are symmetric in appearance to the left side views of FIGS. 5 and 6, respectively.