[5.x] Add @default support to Antlers content allowlists (#14440)

Author: jasonvarga

config/antlers.php

@@ -57,4 +57,25 @@
 
     ],
 
+    /*
+    |--------------------------------------------------------------------------
+    | User content allowlists
+    |--------------------------------------------------------------------------
+    |
+    | These control which tags and modifiers will be permitted in user-supplied
+    | Antlers (e.g. fields with `antlers: true`). Include the literal string
+    | `@default` in the array to merge Statamic's defaults with your own.
+    |
+    */
+
+    // 'allowedContentTags' => [
+    //     '@default',
+    //     'foo:*',
+    // ],
+
+    // 'allowedContentModifiers' => [
+    //     '@default',
+    //     'foo'
+    // ],
+
 ];