In order to publish the demo page (T382414), we need to build it in a form that can be easily published.

• Create a composer command that builds the demo page, but does not start the HTTP server (i.e. the first half of composer start-sandbox, but not the second half)
• Build the sandbox into a self-contained directory that doesn't refer to directories outside of it (i.e. doesn't use ../)
  • This will require changing the way the demo accesses the Codex-PHP library and its dependencies, because right now it does that with require_once '../vendor/autoload.php';
  • Static files from node_modules should be copied over, so that node_modules isn't accessed directly. We don't want to include the entire node_modules directory in what's published, for both bulk and security reasons.