Improve performance of scanning source files

[RobinMalfait]

This PR improves scanning files by scanning chunks of the files in parallel. Each chunk is separated by new lines since we can't use whitespace in classes anyway.

This also means that we can use the power of your CPU to scan files faster. The extractor itself also has less state to worry about on these smaller chunks.

On a dedicated benchmark machine: Mac Mini, M1, 16 GB RAM

❯ hyperfine --warmup 15 --runs 50 \
  -n NEW 'bun --bun /Users/ben/github.com/tailwindlabs/tailwindcss/packages/@tailwindcss-cli/src/index.ts -i ./tailwind.css -o out.css' \
  -n CURRENT 'bun --bun /Users/ben/github.com/tailwindlabs/tailwindcss--next/packages/@tailwindcss-cli/src/index.ts -i ./tailwind.css -o out.css'
Benchmark 1: NEW
  Time (mean ± σ):     337.2 ms ±   2.9 ms    [User: 1376.6 ms, System: 80.9 ms]
  Range (min … max):   331.0 ms … 345.3 ms    50 runs

Benchmark 2: CURRENT
  Time (mean ± σ):     730.3 ms ±   3.8 ms    [User: 978.9 ms, System: 78.7 ms]
  Range (min … max):   722.0 ms … 741.8 ms    50 runs

Summary
  NEW ran
    2.17 ± 0.02 times faster than CURRENT

On a more powerful machine, MacBook Pro M1 Max, 64 GB RAM, the results look even more promising:

❯ hyperfine --warmup 15 --runs 50 \
  -n NEW 'bun --bun /Users/robin/github.com/tailwindlabs/tailwindcss/packages/@tailwindcss-cli/src/index.ts -i ./tailwind.css -o out.css' \
  -n CURRENT 'bun --bun /Users/robin/github.com/tailwindlabs/tailwindcss--next/packages/@tailwindcss-cli/src/index.ts -i ./tailwind.css -o out.css'
Benchmark 1: NEW
  Time (mean ± σ):     307.8 ms ±  24.5 ms    [User: 1124.8 ms, System: 187.9 ms]
  Range (min … max):   291.7 ms … 397.9 ms    50 runs

Benchmark 2: CURRENT
  Time (mean ± σ):     754.7 ms ±  27.2 ms    [User: 934.9 ms, System: 217.6 ms]
  Range (min … max):   735.5 ms … 845.6 ms    50 runs

  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.

Summary
  NEW ran
    2.45 ± 0.21 times faster than CURRENT

Note: This last benchmark is running on my main machine which is more "busy" compared to my benchmark machine. Because of this I had to increase the --runs to get statistically better results. There is still a warning present, but the overall numbers are still very promising.

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These benchmarks are running on our Tailwind UI project where we have >1000 files, and >750 000 lines of code in those files.

Before	After

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I am sure there is more we can do here, because reading all of these 1000 files only takes ~10ms, whereas parsing all these files takes ~180ms. But I'm still happy with these results as an incremental improvement.

For good measure, I also wanted to make sure that we didn't regress on smaller projects. Running this on Catalyst, we only have to deal with ~100 files and ~18 000 lines of code. In this case reading all the files takes ~890µs and parsing takes about ~4ms.

Before	After

Not a huge difference, still better and definitely no regressions which sounds like a win to me.

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Edit: after talking to @thecrypticace, instead of splitting on any whitespace we just split on newlines. This makes the chunks a bit larger, but it reduces the overhead of the extractor itself. This now results in a 2.45x speedup in Tailwind UI compared to 1.94x speedup.