scope_guard/modules: Rename scope_guard to ScopeGuard

Using pascal case for the class name keeps it more consistent with the
majority of the other class names.

src/modules/custom.cpp

@@ -59,7 +59,7 @@ void waybar::modules::Custom::continuousWorker() {
   }
   thread_ = [this, cmd] {
     char* buff = nullptr;
-    waybar::util::scope_guard buff_deleter([buff]() {
+    waybar::util::ScopeGuard buff_deleter([buff]() {
       if (buff) {
         free(buff);
       }