Feat: add support for square millimeter units

src/UnitConverter/Unit/Area/SquareMillimeter.php

@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * This file is part of the jordanbrauer/unit-converter PHP package.
+ *
+ * @copyright 2017 Jordan Brauer <[email protected]>
+ * @license MIT
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types = 1);
+
+namespace UnitConverter\Unit\Area;
+
+use UnitConverter\Measure;
+use UnitConverter\Unit\{ AbstractUnit, UnitInterface };
+
+/**
+ * Square millimeter data class.
+ *
+ * @version 1.0.0
+ * @since 1.0.0
+ * @author Jordan Brauer <[email protected]>
+ */
+abstract class SquareMillimeter extends AreaUnit
+{
+  protected $name = "square millimeter";
+
+  protected $symbol = "mm2";
+
+  protected $units = 0.000001;
+}