feat(discord): add mention_only config for @-mention trigger (#529)

When mention_only is true, the bot only responds to messages that
@-mention the bot. Other messages in the guild are silently ignored.
Also strips the bot mention from content before processing.

Co-authored-by: Will Sarg <12886992+willsarg@users.noreply.github.com>

src/config/mod.rs

@@ -37,6 +37,7 @@ mod tests {
             guild_id: Some("123".into()),
             allowed_users: vec![],
             listen_to_bots: false,
+            mention_only: false,
         };
 
         let lark = LarkConfig {