std: support detached child processes

[izaakschroeder]

Does what it says on the box.

TODO:

• Test manually on macOS
• Test manually on Windows
• Test manually on Linux
• Test suite locally on macOS
• Test suite locally on Windows
• Test suite locally on Linux
• Test failure cases and map any expected errors to something useful

Testing

Existing tests ran via:

mkdir -p build
cd build
cmake .. -DZIG_STATIC_LLVM=ON -DCMAKE_PREFIX_PATH="$(brew --prefix llvm);$(brew --prefix zstd)"
make install
stage3/bin/zig build -p stage4 -Denable-llvm -Dno-lib
stage4/bin/zig build test-std -Dskip-release

Verified manually on macOS 14.3.1 (23D60).

Refs: #19205

[izaakschroeder]

@andrewrk this is my first PR 😄 not really sure on the process or if there's anything else you'd like me to do? I don't have ready access to a Windows machine so that might be a bit hard for me to test.

[Vexu]

Since std.process.Child is a cross-platform API you could write a test for it that you confirm works on macOS and then let the CI test it on Windows and Linux.