Support Attribute-based Plugins

Problem/Motivation

Drupal is moving towards Attribute-based Plugins → . The module currently only supports Annotation-based plugins.

Proposed resolution

The PreprocessorsPluginManager class needs to be updated to wrap the new discovery class that supports backwards compatibility.

Remaining tasks

* Convert PreprocessorsPluginManager to use AttributeDiscoveryWithAnnotations
* Test an Attribute-based plugin.
* Test backwards compatibility with Annotation-based plugin
* Confirm that YML discovery still works as well
* Update documentation - https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... →