feat(er): add UK attribute constraint

Any attribute can now be PK, FK or UK

demos/er.html

@@ -57,6 +57,20 @@ erDiagram
         number final_price
       }
     </pre>
+    <hr />
+
+    <pre class="mermaid">
+    erDiagram
+      "HOSPITAL" {
+        int id PK
+        int doctor_id FK
+        string address UK
+        string name
+        string phone_number
+        string fax_number
+      }
+    </pre>
+    <hr />
 
     <script src="./mermaid.js"></script>
     <script type="module">