jsapi-util: Use JS_PutEscapedString()

In gjs_string_readable(), instead of doing our own escaping of strings,
we can use JS_PutEscapedString(). Since JS_GetStringCharsAndLength() is
going away in SpiderMonkey 38, we have to change this code anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=776966