Retrieve normal message content for mod compatibility (#48)

I discovered that `unsignedContent` was returning `null` on my
server, which causes mixins downstream (namely those used by
Chat Control by declipsonator) to throw NullPointerExceptions.

Using getContent() instead fixes this! This change allows seamless
integration with Chat Control and maybe other chat mods.

Co-authored-by: Variadicism <[email protected]>

src/main/java/eu/pb4/styledchat/other/StyledChatSentMessage.java

@@ -33,7 +33,7 @@ public interface StyledChatSentMessage extends SentMessage, ExtendedSentMessage
 
     record Chat(SignedMessage message, Text override, MessageType.Parameters parameters, RegistryKey<MessageType> sourceType, MutableObject<MessageType.Parameters> colorless) implements StyledChatSentMessage {
         public Text content() {
-            return message.unsignedContent();
+            return message.getContent();
         }
 
         @Override
@@ -75,7 +75,7 @@ public StyledChatSentMessage reformat(MessageType.Parameters pars, RegistryKey<M
 
     record System(SignedMessage message, Text override, MessageType.Parameters parameters, RegistryKey<MessageType> sourceType, MutableObject<MessageType.Parameters> colorless) implements StyledChatSentMessage {
         public Text content() {
-            return this.message.unsignedContent();
+            return this.message.getContent();
         }
 
         @Override