composer.json - Switch to composer-compile-lib. Add more helpers.

The overall effect of this change is to get access to a few more helpers.

Before: Load `composer-compile-plugin`, `scssphp`, and `php-autoprefxer` as
three distinct packages.

After: Load `composer-compile-lib`, which transitively requires all those others
*and* includes some helpers.

Sea also

* https://github.com/civicrm/composer-compile-plugin
* https://github.com/civicrm/composer-compile-lib