Implement P3481

I have almost finished implementing [P3481](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3481r5.html). However, `libc++`'s support for `execution policies` is currently still `experimental`. How should we proceed?
See https://github.com/Cra3z/execution/commit/b8647dc4f82280698434d01d49feb9ff9bf7cb03