test: convert config.test.ts to ESM

[rossiam]

• convert the last test currently running on the yargs branch to work with ESM
• added a few more tests to config.test.ts to get more complete coverage
• updated the HTML output for tests to use dark mode (unfortunately, this doesn't cover the coverage reports which are blinding me 😄)

[changeset-bot]

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Latest commit: b0d070c

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