feat: smart secret detection looks for high entropy level

[aitchiss]

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Summary

Fixes https://linear.app/netlify/issue/WRFL-2597/check-the-shannon-entropy-of-detected-secrets

After running the scan passively against a wide range of repos, we were finding a lot of false positives for example keys baked into dependency READMEs etc. To help eliminate these, after finding the potential secret with our regex, we'll also check that the string has a fairly high shannon entropy score. I've set this at a moderately high level of 4.5 for now which seems like it should be a good threshold to confidently say "we think this is a secret"

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.