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;
+    }
+};

mocks/include/mocks/EmptyApplication.hpp

@@ -228,6 +228,13 @@ public:
         return nullptr;
     }
 
+    IStreamerMode *getStreamerMode() override
+    {
+        assert(false && "EmptyApplication::getStreamerMode was called without "
+                        "being initialized");
+        return nullptr;
+    }
+
 private:
     Paths paths_;
     Args args_;