The ornamental design for a cooker, as shown and described.
FIG. 1 is a front, top left side perspective view of a cooker showing the new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The long dash-short dash broken lines represent boundaries of the claimed design and form no part thereof. All other broken lines represent portions of the cooker that form no part of the claimed design.