feat: allow to scan secrets without buffering whole lines

[pieh]

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Summary

Possible fix for https://linear.app/netlify/issue/FRB-1778/oom-during-ef-bundling (it's not really confirmed, other than logs pointing to secret scanning being a problem and buffering whole very large lines seems like only thing that I managed to reproduce OOM problems with)

For review, I suggest checking commits 1 by 1 as well as toggling "Ignore whitespace changes" in diff viewer.

Description

This adds a new scanning method that is not relying on buffering whole lines in scanned files (through node:readline) and instead allow to operate on smaller chunks of stream ( max length of secrets to scan + whatever size stream.on('data') would result in)

Because this is new method I want to be careful and introduce feature flag that opt into new search algorithm (feature flag not yet created in devcycle, because I don't really like secret_scanning_no_readline name (inverted boolean making it confusing) but didn't come up with better name yet. Assuming no problems are discovered as feature flag is rolled out, this should be cleaned up from the code.

I did run some benchmarks against netlify-react-ui build dir ( 48459 files to scan there) using https://gist.github.com/pieh/a3eccd67cd0c27017acbbf18a87a1c55 and the results are comparable so it should not result in performance regression:

readline x 0.25 ops/sec ±1.29% (31 runs sampled)
NO readline x 0.28 ops/sec ±0.87% (31 runs sampled)
Fastest is NO readline

Goal of the change was not to improve performance, but rather allow secret scanning to work with less available memory and allow Node's garbage collection to function without causing perf regression.

Tests

• (first commit) I converted all snapshot tests for secret scanning to explicit assertions (needed to run same tests in multiple variants without causing problems and just make it more explicit on what actually is being asserted)
• (second commit) Added new test (and fixture) with purposefully limited heap size which does fail with OOM error that does look like same error as on customer builds (that does not guarantee it's same problem, as all OOM errors would look similar)
• (third commit) together with new implementation I made test setup run all existing tests with both implementations and made test added in second commit only run with new implementation (as it will never pass with current implementation)

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Consider converting them to TypeScript.