This amazing new modifier moves vertices on the X axis by some number that you specify. I know, pretty amazing!
Although I know modifiers are not cool anymore since all the cool kids are making nodes now, this patch shows the minimum required code to implement a modifier. I wanted something like this to exist for a long time, so now that I got this far I thought I'd put it here in case anyone else will one day find this useful or interesting.
Thanks to Julian Eisel and Bastien Montagne for helping!