add support for non-customizable signatures (`final`)

[atomgalaxy]

We probably want to enable certain signatures to not be customizable and take precedence over any customizations.

One possibility is then.

virtual auto then(sender auto&&, auto && f) final
  requires is_cpo_customized<get_completion_scheduler<set_value_t>, decltype(s)>
  {
      return then(fwd(s), get_completion_scheduler<set_value_t>(s), fwd(f));
  }
}
virtual auto then(sender auto&&, scheduler auto&& sch, auto && f) = 0;

[lewissbaker]

The branch https://github.com/atomgalaxy/wg21-cust-points/tree/corentin/update_syntax contains the addition of support for final.

[lewissbaker]

Added in R1