Add rule to make id / label required when sssom literal present

LinkML validation passes for this, but not sure it actually does anything.

src/sssom_schema/schema/sssom_schema.yaml

@@ -667,6 +667,24 @@ classes:
     - other
     - comment
     class_uri: owl:Axiom
+    rules:
+      - preconditions:
+          slot_conditions:
+            subject_type: 
+              equals_string: "sssom literal"
+        postconditions:
+          slot_conditions:
+            subject_label:
+              required: true
+      - preconditions:
+          slot_conditions:
+            subject_type:
+              none_of:
+                equals_string: "sssom literal"
+        postconditions:
+          slot_conditions:
+            subject_id:
+              required: true
   mapping registry:
     description: A registry for managing mapping sets. It holds a set of 
       mapping set references, and can import other registries.