The ornamental design for a chairshell, as shown and described.
FIG. 1 is a front perspective view of a chair shell showing the new design;
FIG. 2 is a right side view, the left side view being a mirror image thereof;
FIG. 3 is a front view;
FIG. 4 is a rear view;
FIG. 5 is a top view; and,
FIG. 6 is a bottom view.
The broken lines shown in the drawings are for the purpose of illustrating portions of the article and form no part of the claimed design. The stippling in the drawings represents shading and not a surface pattern.