Move make_robot_particle() to lualib

[Maxreader]

As of 0.18, flying robots got some fancy new death animations. Unfortunately, any robots that are added or retextured by mods will not get this treatment automatically. Each mod that wants to use the "make_robot_particle()" function to generate those death animations currently has to copy those lines of code from base, as it is not accessible currently. I've also noticed that it is duplicated in both the demo and full versions of "flying-robot.lua".

As such, I propose that it be moved to lualib, such that it would no longer need to be duplicated into mod files, as well as removing duplicate code from base.