Move ChatSettings commands to their own file (#4116)

* Move ChatSettings commands to their own file

* reformat error message strings

* move ChatCommands together in CommandController.cpp

* Allow CommandContext to be provided for builtin functions

using variants MEGADANK

* add missing include

* rename to ComandFunctionVariants

also include some move magic & const reffing

src/controllers/commands/CommandContext.hpp

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "common/Channel.hpp"
+#include "providers/twitch/TwitchChannel.hpp"
+
+#include <QStringList>
+
+namespace chatterino {
+
+struct CommandContext {
+    QStringList words;
+
+    // Can be null
+    ChannelPtr channel;
+
+    // Can be null if `channel` is null or if `channel` is not a Twitch channel
+    TwitchChannel *twitchChannel;
+};
+
+}  // namespace chatterino