feat(cli): add detailed non-prod build environment check for Convex deploy (#28)

[Aman1506-web]

Summary

Improved the isNonProdBuildEnvironment function and the related check inside deploy.ts to provide clearer and more specific messages when a non-production environment is detected.

Changes

• Updated isNonProdBuildEnvironment() in envvars.ts to return a descriptive message instead of a boolean.
• Enhanced deploy.ts to display that message when the CLI detects a non-production environment during deployment.
• Now users can see exactly which environment variable caused the warning.

Example Output

Detected VERCEL because the VERCEL env var is set, but VERCEL_ENV is "preview" instead of "production".

Detected a non-production build environment and "CONVEX_DEPLOY_KEY" for a production Convex deployment.
This is probably unintentional.

Closes

Fixes #28

Testing

Verified locally by simulating:

VERCEL=true VERCEL_ENV=preview node dist/cjs/cli/lib/envvars.js
NETLIFY=true CONTEXT=dev node dist/cjs/cli/lib/envvars.js
Confirmed correct descriptive messages for both environments.

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Thanks for maintaining this great project!
Excited to contribute to improving developer experience.

[Aman1506-web]

Fixed Prettier formatting and verified build locally.
Ready for workflow approval.