📎 Add a cli argument to limit worker thread count

[dyc3]

Description

Add a cli flag to set the limit of threads Biome uses to do work. This should only be applicable when using the ci command.

Suggested flag names: --threads, --workers

Discussed briefly in #5097. Note that the environment variable RAYON_NUM_THREADS can already do this, but this is a feature of rayon, not something we own ourselves, and therefore not really discoverable by users.

[arendjr]

This should only be applicable when using the ci command.

What’s the rationale behind this restriction? I could very well see a desire to add the same feature to the LSP/daemon to prevent overloading someone’s workstation, for instance. And the same reasoning could be applied for running individual non-CI commands too.

Of course it’s fine if we simply tackle the CI use case first if that’s less work.

[ematipico]

Until now, four people requested such a feature, and it was always for CI environments with limited resources.

Since the request was coming from a specific need, I feel it makes sense to implement it in CI only, because we can justify it for a real case requested by users.

At the moment, since no user requested the feature for "not overloading the workstation", I think it doesn't make sense to implement it because we can't justify a real, requested use case. (The WASM distribution falls back to one thread by default I think)