ast: Adding rule_head_refs capabilities feature flag (#6346)

To signal/toggle support for general refs in rule heads.

Fixes: #6334
Signed-off-by: Johan Fylling <johan.dev@fylling.se>

capabilities.json

@@ -4705,6 +4705,7 @@
     }
   ],
   "features": [
-    "rule_head_ref_string_prefixes"
+    "rule_head_ref_string_prefixes",
+    "rule_head_refs"
   ]
 }