feat(module): generate configuration for globals

[antfu]

This PR make the module generate a flat config item, register globals from unimport so eslint would expect that auto-import to be presented. So rules like no-undef would not complain

• fix Eslint with Nuxt3 auto-import #309
• fix Error: with @nuxt/eslint-config eslint marks composables as "undefined" when used inside another composable #314
• fix fix: do not throw error for undefined auto-imports in composables #315
• fix @nuxt/eslint-config is not picking up Nuxt globals in JS files #321

[antfu]

Going to merge this and try how it works - later we might want an option to disable that addon for non auto-imported env