Add `is` and `as` support for `std::expected` (v2) #788

Job	Run time
ubu-24 \| g++-14 \| c++2b \| libstdc++ \| ubuntu-24.04	5m 53s
ubu-24 \| g++-13 \| c++2b \| libstdc++ \| ubuntu-24.04	6m 16s
ubu-20 \| g++-10 \| c++20 \| libstdc++ \| ubuntu-20.04	4m 59s
ubu-24 \| clang++-18 \| c++20 \| libstdc++ \| ubuntu-24.04	7m 17s
ubu-24 \| clang++-18 \| c++23 \| libc++-18-dev \| ubuntu-24.04	7m 41s
ubu-22 \| clang++-15 \| c++20 \| libstdc++ \| ubuntu-22.04	5m 30s
ubu-22 \| clang++-15 \| c++20 \| libc++-15-dev \| ubuntu-22.04	5m 29s
ubu-20 \| clang++-12 \| c++20 \| libstdc++ \| ubuntu-20.04	5m 8s
mac-14 \| clang++ \| c++2b \| default \| macos-14	2m 21s
mac-13 \| clang++ \| c++2b \| default \| macos-13	4m 56s
mac-13 \| clang++-15 \| c++2b \| default \| macos-13	7m 6s
win-22 \| cl.exe \| c++latest \| default \| windows-2022	4m 13s
win-22 \| cl.exe \| c++20 \| default \| windows-2022	4m 1s
aggregate-results	2s
	1h 10m 52s

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