refactor: turn StreamerMode into a singleton(-like thing) (#5216)

mocks/include/mocks/DisabledStreamerMode.hpp

@@ -0,0 +1,12 @@
+#pragma once
+
+#include "singletons/StreamerMode.hpp"
+
+class DisabledStreamerMode : public chatterino::IStreamerMode
+{
+public:
+    bool isEnabled() const override
+    {
+        return false;
+    }
+};