Introduce dedicated I/O threadpool for Bun.build on macOS & Windows

[Jarred-Sumner]

What does this PR do?

❯ hyperfine "bun build --outdir=dist ./src/index.jsx --minify --sourcemap" "bun-1.2.3 build --outdir=dist ./src/index.jsx --minify --sourcemap" --warmup=10
Benchmark 1: bun build --outdir=dist ./src/index.jsx --minify --sourcemap
  Time (mean ± σ):     375.6 ms ±   5.0 ms    [User: 499.3 ms, System: 657.6 ms]
  Range (min … max):   370.1 ms … 383.2 ms    10 runs

Benchmark 2: bun-1.2.3 build --outdir=dist ./src/index.jsx --minify --sourcemap
  Time (mean ± σ):     653.9 ms ±   9.6 ms    [User: 581.8 ms, System: 5420.3 ms]
  Range (min … max):   637.5 ms … 668.6 ms    10 runs

Summary
  bun build --outdir=dist ./src/index.jsx --minify --sourcemap ran
    1.74 ± 0.03 times faster than bun-1.2.3 build --outdir=dist ./src/index.jsx --minify --sourcemap

On the bun init shadcn app:

❯ hyperfine "bun ./build.ts" "bun-1.2.3 ./build.ts"
Benchmark 1: bun ./build.ts
  Time (mean ± σ):     156.8 ms ±   2.1 ms    [User: 364.6 ms, System: 462.6 ms]
  Range (min … max):   153.6 ms … 161.6 ms    18 runs

Benchmark 2: bun-1.2.3 ./build.ts
  Time (mean ± σ):     176.1 ms ±   2.9 ms    [User: 351.9 ms, System: 776.0 ms]
  Range (min … max):   172.5 ms … 182.2 ms    17 runs

Summary
  bun ./build.ts ran
    1.12 ± 0.02 times faster than bun-1.2.3 ./build.ts

How did you verify your code works?

Existing tests should cover this. Left two runtime feature flags in to explicitly turn on/off this behavior, so that we can later write more specific tests if needed.

[robobun]

^{Updated 10:22 PM PT - Feb 22nd, 2025}

❌ @Jarred-Sumner, your commit 929d479 has 1 failures in Build #12104:

• test/cli/install/bun-install-registry.test.ts - 1 failing on 🪟 2019 x64-baseline

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🧪   try this PR locally:

bunx bun-pr 17577