[nostalgia/core] Add ticksMs() function

src/nostalgia/core/core.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 - 2019 gtalent2@gmail.com
+ * Copyright 2016 - 2020 gary@drinkingtea.net
  *
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -20,4 +20,8 @@ namespace nostalgia::core {
 
 [[nodiscard]] ox::Error run(Context *ctx);
 
+// Returns the number of milliseconds that have passed since the start of the
+//  program.
+[[nodiscard]] uint64_t ticksMs();
+
 }