[3.2] Support direct loading with a string

[GrahamCampbell]

Example usage, where $content is a string containing the content of some .env file:

<?php

use Dotenv\Environment\Adapter\ArrayAdapter;
use Dotenv\Environment\DotenvFactory;
use Dotenv\Loader;

$loader = new Loader([], new DotenvFactory([new ArrayAdapter()]));

$variables = $loader->loadDirect($content));

Why do we need the array adapter? This allows nested variables to work correctly, but avoid mutating your actual environment.'