`if let` support about mlstyle.jl

[Roger-luo]

I prefer when...

[thautwarm]

https://doc.rust-lang.org/book/first-edition/if-let.html

[findmyway]

https://clojuredocs.org/clojure.core/if-let

[findmyway]

https://clojuredocs.org/clojure.core/when-let

[thautwarm]

@findmyway I think when-let could be replaced by an if-let without else clause. The concept of if-let in Clojure might be better than Rust, I think, for it can accept not only a term of pattern matching but a normal bool expression is also valid.

So what's your ideal if-let syntax in Julia? @Roger-luo prefers keyword when, and so am I, but I'd admit that this could be personal preference(maybe if causes the aesthetic fatigue for its frequently regular usage).

[findmyway]

If that is the case, when will be fine.