This PR will
- allow nextjs export to use all available CPU cores for rendering & writing pages by using child_process
- make use of async-sema to allow each thread to concurrently write multiple paths
- show a fancy progress bar while processing pages (with non-TTY fallback for CI web consoles)
The performance gain for my MacBook with 4 CPU cores went from ~25 pages per second to ~75 pages per second. Beefy CI machines with lots of cores should profit even more.