feat: upgrade to unenv v2

[pi0]

unenv provides Node.js API compatibility for deployment presets without Node.js support.

This PR migrated unenv to v2 with a lot of improvements:

• All polyfills have ESM-compatible exports
• Polyfill support surface improved with less blink mocks

Cloudflare team had been collaborating on v2 and initially adopted unenv v2 for wrangler tooling (announcement) however due to incompatibility of polyfills and alignments needed to make sure we can safely enable node.js compatibility for all Nitro users (see #2949 and #3064 for some context) it took a long time until we could migrate.

TODO before merge:

• release h3 v1 with node-mock-http migration (context: Rollup config breaks some third-party libraries #3071)
• release new v2 rc tag for unenv

Note

More tests will be done on nightly channel before final nitro 2.11 release. If you have relatively big projects deployed to cloudflare (without prerendering) testing before release would be so much helpful to make sure there are no major regressions 🙏🏼