fix: use `Object.defineProperty` for Headers proxy

[kettanaito]

• Propagate from https://github.com/mswjs/interceptors/pull/638
• Fixes https://github.com/mswjs/msw/issues/2281
• Fixes https://github.com/mswjs/msw/issues/2276

[kettanaito]

Released: v2.4.7 🎉

This has been released in v2.4.7!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i msw@latest) to get the newest features and bug fixes.

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