fix(v2): fix theme array deduplication bug

[slorber]

Motivation

Due to some weird reason, the array dedupication logic using [...new Set()] is badly processed and buggy in the production build. Switching to Array.from(new Set()) fixes the problem.

#4972 (comment)

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

[netlify]

✔️ [V2]

🔨 Explore the source changes: 22ebf4f

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60cc6a74b2d0e00007405018

😎 Browse the preview: https://deploy-preview-5000--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	56
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5000--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 624 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.6 kB	0 B
website/build/assets/js/main.********.js	445 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	63.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	28.1 kB	0 B

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