Add [[nodiscard]] to tl::expected

[Quuxplusone]

Microsoft STL added [[nodiscard]] on 2024-12-13; see https://github.com/microsoft/STL/pull/5174

martinmoene/expected-lite added [[nodiscard]] on 2024-12-10; see https://github.com/martinmoene/expected-lite/pull/74

See https://quuxplusone.github.io/blog/2024/12/08/should-expected-be-nodiscard/ for further rationale, if needed.