[execution.syn] Add \libconcept for sender and scheduler (#7176)

Author: hewillk

source/exec.tex

@@ -558,7 +558,7 @@
   template<class Sndr, class Env>
     concept @\exposconcept{single-sender}@ = @\seebelow@; // \expos
 
-  template<sender Sndr>
+  template<@\libconcept{sender}@ Sndr>
     using tag_of_t = @\seebelow@;
 
   // \ref{exec.snd.transform}, sender transformations
@@ -597,7 +597,7 @@
   inline constexpr schedule_t schedule{};
   inline constexpr unspecified read{};
 
-  template<scheduler Sndr>
+  template<@\libconcept{scheduler}@ Sndr>
     using schedule_result_t = decltype(schedule(declval<Sndr>()));
 
   // \ref{exec.adapt}, sender adaptors