fix(turbopack-node): make process_pool inert on wasm
The child-process pool needs tokio::process and a TCP listener, neither of which
exists on wasi. process_pool is a default feature of both turbopack-node and
next-core, so it cannot simply be turned off by a consumer; gate its code on the
target instead, leaving worker_pool — which runs Node worker threads through napi
— as the only backend on wasm.
Co-authored-by: Luke Sandberg <210140+lukesandberg@users.noreply.github.com>