Yargs: separate demand into demandOption and demandCommand

[lrlna]

As @ljharb was fixing #431 with #438 , we've discovered a bit of a black hole that is demand.

The suggested solution at this point is to work off demand being separated in to two different functions: demandOption and demandCommand, clear detailed in this comment.

I am currently working on separating the two. To avoid breaking too many things, I am hoping to keep both require and demand as a alias for demandOption until we decide to completely get rid of demand.

Let's use this thread to track any discussion we might have about demand and the proposed separation.

[kumar303]

This sounds good to me. I was pointed this way because I was trying to do demand(0, 0) to throw an error when extraneous arguments are provided. I agree that demand() could do with a split.

It's sort of a nit-pick but I would call demandCommand() demandArg() instead because not all arguments are commands (they could be file paths, etc).

[lrlna]

@bcoe @nexdrew @maxrimue you _won't_ believe this, buttttttt I actually had time to work this separation out this weekend. lmk if y'all think this might still be useful, and I'll PR a WIP branch before I do anything else \o/

[maxrimue]

@lrlna That's awesome 🎉! I do think this still would be useful, so go ahead!