Add a dedicated `lisp-if-not` / `lif-not` macro

This is in parallel to `if` / `if-not` (so not without precedent). :)

There are a few things here I wasn't completely sure about, like the blurb in the docs and the copied/swapped tests (that seemed like kind of an ugly thing to do, even if it's effective).

Of course, I'm quite happy to amend as requested, or even just close this out if it's something nobody else likes! :smile: