Add support for dev-only nodes. (#12106)

When a node is declared as dev-only, it doesn't show in the default UI
unless the dev mode is enabled in the settings. The intention is to
allow nodes related to unit testing to be included in ComfyUI
distributions without confusing the average user.

server.py

@@ -679,6 +679,8 @@ class PromptServer():
                 info['deprecated'] = True
             if getattr(obj_class, "EXPERIMENTAL", False):
                 info['experimental'] = True
+            if getattr(obj_class, "DEV_ONLY", False):
+                info['dev_only'] = True
 
             if hasattr(obj_class, 'API_NODE'):
                 info['api_node'] = obj_class.API_NODE