Open the directory with Visual Studio Code, and press F5 to launch the Extension Development Host. This will kick off another instance of Visual Studio Code, this one running your prototype extension.

Believe it or not you can use Microsoft's Visual Studio Code from your web browser, or at least, versions of it.

This Visual Studio Code for Java tutorial shows you how to install the Java extensions pack for VS Code, create a Java project and run a simple program.

If you have an existing web application, open it in Visual Studio Code (preferably copying it to a new development branch) and then use the PWA Studio tools to convert it into a PWA.