Add support for rendering highlights with comments

This commit implements rendering comments from the `["comment",
<COMMENT_TEXT>]` tag in a highlight note.

Comment contents get rendered like a kind 1 note's "content" field

This commit also adds the `r` "reference" tag as a standard tag reference type

Changelog-Added: Add support for rendering highlights with comments
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/NdbNote.swift

@@ -335,6 +335,10 @@ extension NdbNote {
     public var referenced_mute_items: References<MuteItem> {
         References<MuteItem>(tags: self.tags)
     }
+    
+    public var referenced_comment_items: References<CommentItem> {
+        References<CommentItem>(tags: self.tags)
+    }
 
     public var references: References<RefId> {
         References<RefId>(tags: self.tags)
@@ -355,6 +359,9 @@ extension NdbNote {
         if known_kind == .dm {
             return decrypted(keypair: keypair) ?? "*failed to decrypt content*"
         }
+        else if known_kind == .highlight {
+            return self.referenced_comment_items.first?.content ?? ""
+        }
 
         return content
     }