Add FinalizationRegistry docs (Revert #22239)

[ketanhwr]

Revert #22239 now that these changes have been released to prod.

[hyperlint-ai]

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PR Change Summary

Reverted changes from pull request #22239 and added documentation for the FinalizationRegistry feature in Workers.

• Added documentation for the FinalizationRegistry feature in Workers.
• Introduced compatibility flag for enabling FinalizationRegistry and WeakRef.
• Provided details on memory efficiency improvements for WebAssembly Workers.

Added Files

• src/content/changelog/workers/2025-05-08-finalization-registry.mdx
• src/content/compatibility-flags/js-weak-refs.md

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