Support nullable arguments and default values for DI container configuration

[clue]

This changeset adds support for optional values in container configuration and support for nullable arguments and default values in factory functions:

<?php

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

$container = new FrameworkX\Container([
    Acme\Todo\UserController::class => function (bool $debug = false) {
        // example UserController class uses $debug, apply default if not set
        return new Acme\Todo\UserController($debug);
    },
    'debug' => true
]);

// …

<?php

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

$container = new FrameworkX\Container([
    Acme\Todo\UserController::class => function (?string $name) {
        // example UserController class uses $name, defaults to null if not set
        return new Acme\Todo\UserController($name ?? 'ACME');
    },
    'name' => 'Demo'
]);

// …

The previous version would always omit parameters with default values and nullable values without checking if an explicit container variable is set. We now first check to see if an explicit container variable is set and use this value if available. If this value is not available, we continue using the default value or a null value. This is the next step in adding better configuration support and support for (optional) environment variables and .env (dotenv) files as discussed in #101.

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