fix: fix shellUtils stubbing with ES modules

[ehmicky]

Part of #749

ES modules make stubbing harder. More precisely, it is not possible to stub the top-level variable being exporter/imported (either named or default), only its properties. This is, unlike CommonJS, ES modules are linked at parse-time, not runtime.

To workaround this, in order to migrate to pure ES modules, this PR wrap shellUtils into an object, since it is being stubbed in tests.

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[github-actions]

⏱ Benchmark results

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• T (current commit): 7.3s

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• T (current commit): 56.8s