move `sender_expr[_for]` concepts out of the STDEXEC namespace (#1906) #2881

Job	Run time
macos-CPU (MacOS) (clang, Release)	3m 53s
CPU (Windows) (msvc, Debug)	19s
macos-CPU (MacOS) (clang, Debug)	4m 3s
CPU (Windows) (msvc, Release)	14m 42s
CPU (clang 16, Debug)	4m 28s
CPU (gcc 11, Release)	3m 51s
CPU (gcc 13, Debug)	3m 59s
CPU (clang 16, Release)	4m 19s
CPU (clang 16, Debug, c++23)	4m 40s
CPU (clang 16, Release, ASAN)	4m 25s
CPU (gcc 11, Release, ASAN)	3m 54s
CPU (gcc 14, Release, LEAK)	4m 40s
CPU (gcc 11, Debug)	4m 1s
CPU (gcc 12, Release, TSAN)	4m 12s
CPU (gcc 14, Release, c++23)	13m 3s
CI (CPU) (MacOS)	4s
CI (CPU)	3s
CI (CPU) (Windows)	0s
	1h 18m 36s

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