Class rename, composer add, phpUnit tests add

Rename the class from ReadEnvFile to DotEnv.
Add everything to a composer json file so it can be published to
composer repository.
Add phpUnit tests for system

vendor/sebastian/environment/src/OperatingSystem.php

@@ -0,0 +1,53 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of sebastian/environment.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace SebastianBergmann\Environment;
+
+use const DIRECTORY_SEPARATOR;
+use const PHP_OS;
+use const PHP_OS_FAMILY;
+use function defined;
+
+final class OperatingSystem
+{
+    /**
+     * Returns PHP_OS_FAMILY (if defined (which it is on PHP >= 7.2)).
+     * Returns a string (compatible with PHP_OS_FAMILY) derived from PHP_OS otherwise.
+     */
+    public function getFamily(): string
+    {
+        if (defined('PHP_OS_FAMILY')) {
+            return PHP_OS_FAMILY;
+        }
+
+        if (DIRECTORY_SEPARATOR === '\\') {
+            return 'Windows';
+        }
+
+        switch (PHP_OS) {
+            case 'Darwin':
+                return 'Darwin';
+
+            case 'DragonFly':
+            case 'FreeBSD':
+            case 'NetBSD':
+            case 'OpenBSD':
+                return 'BSD';
+
+            case 'Linux':
+                return 'Linux';
+
+            case 'SunOS':
+                return 'Solaris';
+
+            default:
+                return 'Unknown';
+        }
+    }
+}