Run clang-format on all files

The main change here is to reorder headers.

src/future_feature_flags.cpp

@@ -1,8 +1,9 @@
 #include "config.h"  // IWYU pragma: keep
 
-#include <cwchar>
 #include "future_feature_flags.h"
 
+#include <cwchar>
+
 /// The set of features applying to this instance.
 static features_t global_features;