Allow Plugin::getCollection() to be used

Add this method to the public API of `Plugin` as it has some use cases
for plugin test case setup.

Refs #12682

src/Core/Plugin.php

@@ -411,7 +411,9 @@ protected static function _includeFile($file, $ignoreMissing = false)
     /**
      * Get the shared plugin collection.
      *
-     * @internal
+     * This method should generally not be used during application
+     * runtime as plugins should be set during Application startup.
+     *
      * @return \Cake\Core\PluginCollection
      */
     public static function getCollection()