Validate extra action keys

src/config/action.rs

@@ -61,6 +61,9 @@ impl<'de> Deserialize<'de> for Action {
                     }
                     None => return serde_error::<Self::Value, M>("missing action"),
                 };
+                if let Some(key) = map.next_key::<String>()? {
+                    return serde_error::<Self::Value, M>(&format!("only one action key is expected but also got: {}", key));
+                }
                 Ok(action)
             }
         }