Dinnerbone just released the very cool auto-complete system that 1.13 has built-in to commands.  They call it Brigadier.  He says we can use it where ever we want.  But he gives no info as to how it can be used.  I would think we'd still need some kind of modding tool like Forge to even be able to interface with the Minecraft game code.  I guess they assume people use a modding tool and then incorporate Brigadier into their mod?

Its a library, you can use it like a normal library. However you SHOULDN'T use it in Minecraft because it is already in Minecraft. 

Once 1.13 Forge gets released you'll have Brigadier at your disposal.

You can take a look at how Forge uses it here: https://github.com/MinecraftForge/MinecraftForge/tree/1.13-pre/src/main/java/net/minecraftforge/server/command