Add flag for `show` plugin to separate fields with a newline

[XVilka]

Something like

type qwe = {
    a: int;
    b: string;
} [@@deriving show { with_path = false, with_newline = true }

will show

{
  a = 4;
  b = "something";
}

[whitequark]

I'm wondering if maybe a dynamically scoped flag would be more fitting there, to give caller control over printing. (And for with_path as well.)

[XVilka]

@whitequark could be the option too, agree.