brookter

It's Karabiner Elements on the Mac.

On Linux you can get it to do the same thing with a combination of xmodmap, setxkbmap and xcape (but it's fiddly) and on Windows with AutoHotkey (but it's fiddly) — neither are as easy to use as Karabiner Elements, but they do work.

The latest version of Elements has supported this for several months, if not longer (I think they'd made it work for High Sierra at least, but I could be wrong about the exact timing.)

I use it for both Caps Lock -> Escape/Control and Return -> Return/Control and it's indispensible.