[execution.syn] Add enable_sender to synopsis

Fixes NB US 199-324 (C++26 CD).

Author: jensmaurer

source/exec.tex

@@ -530,6 +530,9 @@
   // \ref{exec.snd}, senders
   struct @\libglobal{sender_t}@ {};
 
+  template<class Sndr>
+    inline constexpr bool enable_sender = @\seebelow@;
+
   template<class Sndr>
     concept @\libconcept{sender}@ = @\seebelow@;