Add App::__invoke() method to enable custom integrations

[clue]

This changeset adds a new App::__invoke() method to make the App class callable and to enable custom integrations.

This method allows you to pass in a single HTTP request object that will be processed according to any registered routes and middleware and will return an HTTP response object as a result.

$app = new FrameworkX\App();
$app->get('/', fn() => React\Http\Message\Response::plaintext("Hello!\n"));

$request = new React\Http\Message\ServerRequest('GET', 'https://example.com/');
$response = $app($request);

assert($response instanceof Psr\Http\Message\ResponseInterface);
assert($response->getStatusCode() === 200);
assert($response->getBody()->getContents() === "Hello\n");

This is particularly useful for higher-level integration test suites and for custom integrations with other runtime environments like serverless functions or other frameworks. Otherwise, most applications would likely want to use the run() method to run the application and automatically accept incoming HTTP requests according to the PHP SAPI in use.

Builds on top of #235 and #224
Also refs #130 and #185