pim-types: add getter for bday property

src/pim-types.ts

@@ -381,6 +381,10 @@ export class ContactType implements PimType {
     return this.comp.getFirstPropertyValue("n");
   }
 
+  get bday() {
+    return this.comp.getFirstPropertyValue("bday");
+  }
+
   get group() {
     const kind = this.comp.getFirstPropertyValue("kind");
     return kind in ["group", "organization"];