feat: Add `folder-plus` icon

Closes #166

icons/folder-plus.svg

+<svg
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M22 19a2 2 0 0 1-2 2H4a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h5l2 3h9a2 2 0 0 1 2 2z" />
+  <line x1="12" y1="11" x2="12" y2="17" />
+  <line x1="9" y1="14" x2="15" y2="14" />
+</svg>