Add flags.first_message filter variable (#3292)

Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

src/controllers/filters/parser/FilterParser.cpp

@@ -29,6 +29,7 @@ ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel)
      * flags.sub_message
      * flags.system_message
      * flags.reward_message
+     * flags.first_message
      * flags.whisper
      *
      * message.content
@@ -80,6 +81,7 @@ ContextMap buildContextMap(const MessagePtr &m, chatterino::Channel *channel)
         {"flags.system_message", m->flags.has(MessageFlag::System)},
         {"flags.reward_message",
          m->flags.has(MessageFlag::RedeemedChannelPointReward)},
+        {"flags.first_message", m->flags.has(MessageFlag::FirstMessage)},
         {"flags.whisper", m->flags.has(MessageFlag::Whisper)},
 
         {"message.content", m->messageText},

src/controllers/filters/parser/Tokenizer.hpp

@@ -23,6 +23,7 @@ static const QMap<QString, QString> validIdentifiersMap = {
     {"flags.sub_message", "sub/resub message?"},
     {"flags.system_message", "system message?"},
     {"flags.reward_message", "channel point reward message?"},
+    {"flags.first_message", "first message?"},
     {"flags.whisper", "whisper message?"},
     {"message.content", "message text"},
     {"message.length", "message length"}};