test: improve test coverage; multiple internal refactors

[Josh-Cena]

Motivation

In order to improve the test coverage, I had to make a lot of refactors😟

• Some code branches that are overly defensive and can never been reached have been removed
• theme-translations/update.mjs is now ESM. I've managed to split the extractThemeCodeMessages util that the Jest test uses into a separate file, so that we get the best of both worlds
• Some core functions are extracted into utils, so that core/commands is now fully untested except for swizzle. We would add E2E tests later, once we have a separate org where I plan to host E2E fixture repos.
• I've managed to reliably remove the quotes of some route module properties, so that we no longer add quotes all the time.

Have you read the Contributing Guidelines on pull requests?

Yes

[netlify]

✔️ [V2]

🔨 Explore the source changes: 1714a33

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	69
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-6912--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 792 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	49.9 kB
website/build/assets/css/styles.********.css	105 kB
website/build/assets/js/main.********.js	599 kB
website/build/index.html	38.7 kB

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