Icon: New icon for status-neutral (12px)

Purpose

Support the work of gitlab#328245 (closed) by adding a status-neutral icon.

Screenshots

Examples

Grid (12px)

The lines in the icon are 1.5px to align with the forthcoming icon effort. There's a 1px gap between the lines.

Checklist

For a new icon and/or illustration:

• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

Closes #193 (closed)