Role/Capability: Add a new update_role function.

Until now, changing a user's role involved deleting a user's role then re-adding.  This change creates a new `update_role` function and associated method in `WP_Roles` to consolidate this process.

This commit also introduces new unit tests around `update_role` and adds additional "unhappy path" tests for roles and capabilities in general.

Props maksimkuzmin, peterwilsoncc, NomNom99, costdev, SergeyBiryukov.
Fixes #54572.
Built from https://develop.svn.wordpress.org/trunk@54213


git-svn-id: http://core.svn.wordpress.org/trunk@53772 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/capabilities.php

@@ -1030,6 +1030,30 @@ function add_role( $role, $display_name, $capabilities = array() ) {
 	return wp_roles()->add_role( $role, $display_name, $capabilities );
 }
 
+/**
+ * Updates an existing role. Creates a new role if it doesn't exist.
+ *
+ * Modifies the display name and/or capabilities for an existing role.
+ * If the role does not exist then a new role is created.
+ *
+ * The capabilities are defined in the following format: `array( 'read' => true )`.
+ * To explicitly deny the role a capability, set the value for that capability to false.
+ *
+ * @since 6.1.0
+ *
+ * @param string      $role         Role name.
+ * @param string|null $display_name Optional. Role display name. If null, the display name
+ *                                  is not modified. Default null.
+ * @param bool[]|null $capabilities Optional. List of capabilities keyed by the capability name,
+ *                                  e.g. `array( 'edit_posts' => true, 'delete_posts' => false )`.
+ *                                  If null, don't alter capabilities for the existing role and make
+ *                                  empty capabilities for the new one. Default null.
+ * @return WP_Role|void WP_Role object, if the role is updated.
+ */
+function update_role( $role, $display_name = null, $capabilities = null ) {
+	return wp_roles()->update_role( $role, $display_name, $capabilities );
+}
+
 /**
  * Removes a role, if it exists.
  *