Enable global_namespace_import rule (#1)

See https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/2.19/doc/rules/import/global_namespace_import.rst
See codeigniter4/CodeIgniter4#5250

* config: change global_namespace_import to true
* Use explicit options
* Fix CS

src/CodeIgniter4.php

@@ -171,7 +171,11 @@ public function __construct()
                 'fix_inline'     => true,
                 'replacements'   => ['inheritDocs' => 'inheritDoc'],
             ],
-            'global_namespace_import'     => false,
+            'global_namespace_import' => [
+                'import_constants' => false,
+                'import_functions' => false,
+                'import_classes'   => true,
+            ],
             'group_import'                => false,
             'header_comment'              => false, // false by default
             'heredoc_indentation'         => ['indentation' => 'start_plus_one'],