Turbopack: Revert default loader runtime backend to child processes (#90675)
We're getting crashes in node: https://github.com/vercel/workflow/actions/runs/22500511618/job/65186125774#step:10:224
It looks like this should be fixed in newer versions of node:
- https://github.com/napi-rs/napi-rs/issues/2555#issuecomment-3645173223
- https://github.com/nodejs/node/pull/55877
- https://github.com/nodejs/node/pull/61400
- https://github.com/nodejs/node/pull/61661
So we may enable the worker threads feature by default if we detect a new enough version of node or bun.