Refactor recent messages API (#4763)

This exposes internal functions for testing by splitting the implementation & internal API into separate files

src/providers/recentmessages/Impl.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include "common/Channel.hpp"
+#include "messages/Message.hpp"
+
+#include <IrcMessage>
+#include <QJsonObject>
+#include <QString>
+#include <QUrl>
+
+#include <memory>
+#include <vector>
+
+namespace chatterino::recentmessages::detail {
+
+// convertClearchatToNotice takes a Communi::IrcMessage that is a CLEARCHAT
+// command and converts it to a readable NOTICE message. This has
+// historically been done in the Recent Messages API, but this functionality
+// has been moved to Chatterino instead.
+Communi::IrcMessage *convertClearchatToNotice(Communi::IrcMessage *message);
+
+// Parse the IRC messages returned in JSON form into Communi messages
+std::vector<Communi::IrcMessage *> parseRecentMessages(
+    const QJsonObject &jsonRoot);
+
+// Build Communi messages retrieved from the recent messages API into
+// proper chatterino messages.
+std::vector<MessagePtr> buildRecentMessages(
+    std::vector<Communi::IrcMessage *> &messages, Channel *channel);
+
+// Returns the URL to be used for querying the Recent Messages API for the
+// given channel.
+QUrl constructRecentMessagesUrl(const QString &name);
+
+}  // namespace chatterino::recentmessages::detail