Add :unless loop modifier

[primo-ppcg]

Related: #1286

• Adds :unless loop modifier

> (seq [i :range [0 10] :unless (odd? i)] i)
@[0 2 4 6 8]

[pepe]

LGTM

[sogaiu]

Tests gave good results 👍

Some manual invocations (edited for readability):

$ ./build/janet
Janet 1.30.0-db366558 linux/x64/gcc - '(doc)' for help

repl:1:> (seq [i :range [10] :unless (even? i)] i)
@[1 3 5 7 9]

repl:2:> (loop [i :down-to [5] :unless (even? i)] (print i))
5
3
1
nil