feat!(lexicons/v0.feed.playlistTrack): add required `createdAt` field

packages/lexicons/defs/sh/comet/v0/feed/playlistTrack.json

···
       "key": "tid",
       "record": {
         "type": "object",
-        "required": ["track", "playlist", "position"],
+        "required": ["track", "playlist", "position", "createdAt"],
         "properties": {
           "track": {
             "type": "string",
···
             "type": "string",
             "format": "at-uri"
           },
-          "position": { "type": "integer", "minimum": 0 }
+          "position": { "type": "integer", "minimum": 0 },
+          "createdAt": {
+            "type": "string",
+            "format": "datetime",
+            "description": "Timestamp for when the track entry was originally added to the playlist."
+          }
         }
       }
     }