nip-10: fix duplicate replies in feed

some replies rendered twice in different positions, seems to be
related to deprecated positional event tags and a regression
introduced in a596121821

added 2 new functions to get reply to and replies of an event.
these functions are not pretty but should follow nip-10 with
correct fallback.

example event that had some replies rendered twice:
22e4ea80161ac591059da611d3ab63c583cb1d47a706826db2fc6955ac0a70b5

@@ -697,0 +736,4 @@
+        // nip-10: fallback to deprecated positional 'e' tags
+        return eventTags.at(-1)[1] === evt.id;
+      }
+    );