phpcbf is included when you install PHP_CodeSniffer. Tom McFarlin has written a nice guide on how to install PHP_CodeSniffer and the WordPress Coding Standards Rules for VSCode. If you follow this guide, phpcbf will be installed in the project root, in ./vendor/bin/phpcbf
You'll find the convert.php script at GitHub
Add the content in tasks.json to ./.vscode/tasks.json. Make sure the paths are correct.
When you want to beautify PHP, or convert array() to [], on a Mac, hit Shift+Cmd+B and select the task runner.
I learned about the phpcbf task runner from WraithKenny