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/code-unit/src/exceptions/InvalidCodeUnitException.php

@@ -0,0 +1,16 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of sebastian/code-unit.
+ *
+ * (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\CodeUnit;
+
+use RuntimeException;
+
+final class InvalidCodeUnitException extends RuntimeException implements Exception
+{
+}