fix: add .vercel to .gitignore when running astro add vercel

[VedantMadane]

Summary

When running
px astro add vercel, the .vercel\ directory (which contains build output) is now automatically added to .gitignore\ to prevent build artifacts from being committed.

Changes

• Added handling in the \add\ CLI command to check for Vercel integration
• If .gitignore\ exists, appends .vercel\ entry (if not already present)
• If .gitignore\ doesn't exist, creates one with .vercel\ entry
• Follows the same pattern as Cloudflare integration scaffolding

Testing

• Builds successfully
• Follows existing patterns for cloudflare/tailwind/svelte integrations

Fixes #15058

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7c9b998

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[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

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_{Comparing VedantMadane:fix-vercel-gitignore (7c9b998) with main (27d7ffb)¹}

Footnotes

1. No successful run was found on main (44f4e78) during the generation of this report, so 27d7ffb was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩