Updating to a new Lua release

This is how we maintain our patched Lua branch

1. Switch to upstream branch
   • replace working directory with new Lua that came out
   • commit with message "Upstream 5.x.x"
   • push it
2. Switch to main branch
   • merge upstream
   • there might be trivial merge conflicts in README and lua.h
   • commit with message "Patched 5.x.x"
   • push it
3. Create a new tag and push it