Add new search predicate to enable searching for messages matching a regex (#3282)

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

src/messages/search/RegexPredicate.cpp

@@ -0,0 +1,22 @@
+#include "RegexPredicate.hpp"
+
+namespace chatterino {
+
+RegexPredicate::RegexPredicate(const QString &regex)
+    : regex_(regex, QRegularExpression::CaseInsensitiveOption)
+{
+}
+
+bool RegexPredicate::appliesTo(const Message &message)
+{
+    if (!regex_.isValid())
+    {
+        return false;
+    }
+
+    QRegularExpressionMatch match = regex_.match(message.messageText);
+
+    return match.hasMatch();
+}
+
+}  // namespace chatterino