A Crow app defines an interface to allow the developer access to all the different parts of the framework, without having to manually deal with each one.<br><br>
An app allows access to the HTTP server (for handling connections), router (for handling URLs and requests), Middlewares (for extending Crow), among many others.<br><br>
When using `run_async()`, make sure to use a variable to save the function's output (such as `#!cpp auto _a = app.run_async()`). Otherwise the app will run synchronously.