support RSPACK_WORKER_THREADS to limit CPU core count

[hardfist]

Trying to make migration poc for the large code base and due to some legacy codebase we are experiencing initial build takes some time. It is fine, but at some stage of build rspack seems excessively occupies CPU (or ram) usage makes other system activites feel hang - literally whole UI stops and hourglass icon appears around 20~30sec.

I'm curious if there's a way to set the limit for the CPU core count or similar, so it may take longer but won't block system use while dev waiting for it.

Originally posted by @kwonoj in #8276

[stale]

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