fix: correct css modules hashing to match Webpack when leading digit #8735

[daveskybet]

Summary

As detailed in #8735 Rspack is currently out of sync with a recent change in Webpack around how css module hashes that lead with a digit are handled.

Current Rspack functionality removes all digits from the start of the hash so that hashed classnames never start with a digit, however this produces an edge case where when the hash is entirely digits it is reduced down to an empty string.

Webpack fixed this issue recently to instead prepend hashes starting with a digit with a underscore character ("_") - https://github.com/webpack/webpack/blob/964c0315df0ee86a2b4edfdf621afa19db140d4f/lib/dependencies/CssLocalIdentifierDependency.js#L81

This commit updates the regex/replace functionality to match Webpack's implementation.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[CLAassistant]

All committers have signed the CLA.

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[codspeed-hq]

CodSpeed Performance Report

Merging #8778 will not alter performance

_{Comparing daveskybet:correct_css_module_hashing_8735 (82171fe) with main (52c2309)}

Summary

✅ 1 untouched benchmarks

[ahabhgk]

LGTM, thanks! @LingyuCoder Can you help to run the CI and merge this?