perf: flatten concat_source when constructing it

[Boshen]

Port of https://github.com/webpack/webpack-sources/blob/9f98066311d53a153fdc7c633422a1d086528027/lib/ConcatSource.js#L18-L27

related #72

[Boshen]

Current dependencies on/for this PR:

• main
  • PR perf: flatten concat_source when constructing it #78 👈
    • PR feat: add optimize to ConcatSource #81

This stack of pull requests is managed by Graphite.

[codspeed-hq]

CodSpeed Performance Report

Merging #78 will not alter performance

_{Comparing 11-29-perf_flatten_concat_source_when_constructing_it (bc00dff) with main (8bae04e)}

Summary

✅ 4 untouched benchmarks

[SyMind]

@ahabhgk This PR guarantees stable hashing when the output from the Source map method yields identical results, despite variations in the sequence of ConcatSource aggregations.