Undo removal of Js.Null.to_opt

[glennsl]

Addresses #2515

[TheSpyder]

while you're there would you mind fixing the spelling error in the from_opt deprecation message? ;)
https://github.com/BuckleScript/bucklescript/blob/a543643a63cc1472f11dea953bbdd7b35e133827/jscomp/runtime/js_null.mli#L81

And looking at that code, actually, should this change be @@ocaml.deprecated the way from_opt is rather than just @@deprecated?

[glennsl]

Will do. I don't think the ocaml. namespace matters, but might as well be consistent.