refactor(file-templates): c++-mode/__main.cpp: modernize

- Drops `using namespace std`: a known anti-pattern which imports a
  massive namespace into the global namespace. See
  https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice
- Modernizes/generalizes `main`'s signature with a trailing return
  type (C++11)

Ref: https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice
Close: #7387

modules/editor/file-templates/templates/c++-mode/__main.cpp

@@ -4,9 +4,7 @@
 # --
 #include <iostream>
 
-using namespace std;
-
-int main(int argc, char *argv[]) {
+auto main(int argc, char *argv[]) -> int {
     $0
 
     return 0;