feat: allow entryFileNames, chunkFileNames functions for legacy

[avocadowastaken]

Description

Closes #3636

Additional context

I can't say that this solution will cover all the edge cases.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

Thanks for the PR @umidbekk, this looks good. We should try to test this one on the current legacy playground though instead of creating a new playground, so we keep test suite run times at check. Would you try to refactor the new test there?

[avocadowastaken]

Thanks for the PR @umidbekk, this looks good. We should try to test this one on the current legacy playground though instead of creating a new playground, so we keep test suite run times at check. Would you try to refactor the new test there?

Done. I also found a bug - mutation of fileNames argument was a bad idea, so there are some source changes too.