tweak/negative_dice_support

1d20-1d4+2 type stuff.

cool! Now we can cast d&d 5e spells like Bane (-1d4 to rolls)

Excellent.

While you are there is there any reason not to also add multiplication (and I suppose division, though I can't think of any games that use it) to the operators?

I wonder: Would multiplication be 1d6*20 or 1d6x20?

This would bring up the question of precedence, but I think left to right would make the most sense? 1d6*20+2 obviously does not mean 1d6*22 else they would just have said that, it means 1d6*20 plus an extra 2 to the total. :)

I think that will happen, but I want to make the commits step-by-step.

I was thinking of highest/lowest next.
4d6k3 (4d6 keep three) synonym for 4d6h3 (h for highest).
2d20l1 (2d20 lowest 1) would be DnD 5e disadvantage.

Yes. Drop and Keep, or Highest and Lowest are probably the most useful.