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I re-added the in-game code editor in this latest update (version 1.9.7). The button to open it is under World Settings -> "Open Code Editor". 

Exporting a mod through Unity3d is going to be more reliable and run faster, but writing JavaScript in-game is still novel and fun. I'll continue supporting the in-game editor if there continues to be interest in it. 

Awesome! So much fun.