Fix inconsistent printing of opening extension expressions.

[IwanKaramazow]

Fixes #1978

The open expr open Syntax in the example contains an extension as child expression.
This is a "sequency" expression and should not be formatted with the
open syntax sugar.

The following should be formatted as is:

let work = () => {
  open Syntax;
  let%bind name = x;
  name;
};