refactor(NcTimezonePicker): migrate component to Typescript

[susnux]

☑️ Resolves

• For [FR]: TypeScript support #841

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[ShGKme]

/backport to stable8

[backportbot]

The backport to stable8 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable8
git pull origin stable8

# Create the new backport branch
git checkout -b backport/6821/stable8

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 56623434

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/6821/stable8

Error: Failed to check for changes with origin/stable8: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.