Support loading environment variables from DI container configuration

[clue]

This changeset adds support for loading environment variables from the DI container configuration:

<?php

require __DIR__ . '/../vendor/autoload.php';

$container = new FrameworkX\Container([
    React\MySQL\ConnectionInterface::class => function (string $DB_HOST = 'localhost', string $DB_USER = 'root', string $DB_PASS = '', string $DB_NAME = 'acme') {
        // connect to database defined in optional $DB_* environment variables
        $uri = 'mysql://' . $DB_USER . ':' . rawurlencode($DB_PASS) . '@' . $DB_HOST . '/' . $DB_NAME . '?idle=0.001';
        return (new React\MySQL\Factory())->createLazyConnection($uri);
    }
]);

$app = new FrameworkX\App($container);

// …

The previous version only supported explicit configuration of container variables, we now also support automatically loading all environment variables with all uppercase names. This is the next big step in adding better configuration support and support for environment variables and .env (dotenv) files as discussed in #101.

Builds on top of #183, #182, #181, #180, #179, #178, #163, #97, #95 and others