Support string variables in container configuration for factory functions

[clue]

This changeset adds support for string variables in the container configuration for all factory functions:

<?php

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

$container = new FrameworkX\Container([
    Acme\Todo\UserController::class => function (string $name, string $hostname) {
        // example UserController class requires two string arguments
        return new Acme\Todo\UserController($name, $hostname);
    },
    'name' => 'Acme',
    'hostname' => fn(): string => gethostname()
]);

// …

This is a first step in adding support for environment variables and .env (dotenv) files as discussed in #101.

Builds on top of #163, #97, #95 and others