Add momentary/tap layer/action aka QMK LT()

[b-]

The best way to explain what I'm looking for is to quote QMK documentation for its equivalent LT() feature:

LT(layer, kc) - momentarily activates layer when held, and sends kc when tapped.

So the idea is I would set, e.g., the back button as an input, and the output would be switching to another layer while I hold it down, but if I release it within e.g., 250ms the remapper will issue a back button press.