Warn on the usage of fallback arrays in the exports/imports field

[sapphi-red]

Fallback arrays in the exports/imports field works differently among tools.

• Node.js, Vite, esbuild: only validates the path
• Webpack, TypeScript: check the file existence

To make a package compatible with all tools, I think it should be recommended to avoid using this feature.
https://x.com/atcb/status/1736987593534034184, web-infra-dev/rspack#5052 (comment)

Related issues: nodejs/node#37928, nodejs/node#44282, vitejs/vite#4439, web-infra-dev/rspack#5052, microsoft/TypeScript#50762, webpack/enhanced-resolve#400, lukeed/resolve.exports#17, evanw/esbuild#2974

[sapphi-red]

Webpack (enhanced-resolve) has changed their behavior to align with Node.js: webpack/enhanced-resolve#429