Add shell_write_string_to_stream()

Add shell_write_string_to_stream()

Add a helper function to write a string as UTF-8 to a GOutputStream.
The signature of GOutputStream:

 gboolean g_output_stream_write_all (GOutputStream *stream,
                                     const void *buffer,
                                     gsize count,
                                     gsize *bytes_written,
                                     GCancellable *cancellable,
                                     GError **error);

Can't currently be handled by GJS.

https://bugzilla.gnome.org/show_bug.cgi?id=618189