Handle escape sequences correctly in @as.

[cristianoc]

Context: #5401

[cknitt]

See the difference in the way the string "\h\e\l\lo" is compiled:

external f: (@as("\h\e\l\lo") _, int) => unit = "f"

let x = "\h\e\l\lo"
let y = f(42)

compiles to

// Generated by ReScript, PLEASE EDIT WITH CARE
'use strict';


f("\\h\\e\\l\\lo", 42);

var x = "\h\e\l\lo";

var y;

exports.x = x;
exports.y = y;
/*  Not a pure module */

I.e. when used in an attribute, the backslashes are duplicated.

(see also #5391)