Add [square-]pi/sigma icons (#1278)

* Add `pi-square` icon

* Add `sigma-square` icon

icons/pi-square.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "../icon.schema.json",
+  "tags": [
+    "constant",
+    "code",
+    "coding",
+    "programming",
+    "symbol",
+    "trigonometry",
+    "geometry",
+    "formula"
+  ],
+  "categories": [
+    "development",
+    "maths",
+    "shapes"
+  ]
+}

icons/pi-square.svg

@@ -0,0 +1,16 @@
+<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"
+>
+  <rect width="18" height="18" x="3" y="3" rx="2" />
+  <path d="M7 7h10" />
+  <path d="M10 7v10" />
+  <path d="M16 17a2 2 0 0 1-2-2V7" />
+</svg>

icons/sigma-square.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "../icon.schema.json",
+  "tags": [
+    "sum",
+    "calculate",
+    "formula",
+    "maths",
+    "enumeration",
+    "enumerate"
+  ],
+  "categories": [
+    "text",
+    "maths"
+  ]
+}

icons/sigma-square.svg

@@ -0,0 +1,14 @@
+<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"
+>
+  <rect width="18" height="18" x="3" y="3" rx="2" />
+  <path d="M16 8.9V7H8l4 5-4 5h8v-1.9" />
+</svg>