fix: missing `mp-units/ext/format.h` added to `fixed_string.h` #28

Job	Run time
std::format none C++20 Clang-18 Release	5m 28s
std::format none C++20 Clang-18 Debug	5m 5s
std::format none C++23 Clang-18 Release	5m 15s
std::format none C++23 Clang-18 Debug	4m 54s
std::format gsl-lite C++20 Clang-18 Release	5m 26s
std::format gsl-lite C++20 Clang-18 Debug	4m 56s
std::format gsl-lite C++23 Clang-18 Release	5m 10s
std::format gsl-lite C++23 Clang-18 Debug	5m 1s
std::format ms-gsl C++20 Clang-18 Release	5m 20s
std::format ms-gsl C++20 Clang-18 Debug	4m 58s
std::format ms-gsl C++23 Clang-18 Release	5m 23s
std::format ms-gsl C++23 Clang-18 Debug	4m 57s
fmtlib none C++20 Clang-18 Release	6m 40s
fmtlib none C++20 Clang-18 Debug	6m 14s
fmtlib none C++23 Clang-18 Release	5m 26s
fmtlib none C++23 Clang-18 Debug	5m 55s
fmtlib gsl-lite C++20 Clang-18 Release	6m 41s
fmtlib gsl-lite C++20 Clang-18 Debug	6m 25s
fmtlib gsl-lite C++23 Clang-18 Release	5m 28s
fmtlib gsl-lite C++23 Clang-18 Debug	6m 21s
fmtlib ms-gsl C++20 Clang-18 Release	6m 46s
fmtlib ms-gsl C++20 Clang-18 Debug	6m 20s
fmtlib ms-gsl C++23 Clang-18 Release	5m 33s
fmtlib ms-gsl C++23 Clang-18 Debug	6m 11s
	2h 15m 53s

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