feat(nuxt): skip the root uno.config when merging

[enkot]

Related #3837

#3838 added logic to merge ALL uno.configs from Nuxt layers and generate ./.nuxt/uno.config.mjs.

Problem:
Because Eslint, VSCode plugins and other tools rely on uno.config file, we need to tell them to look for the new location(./.nuxt). While this can be done for Eslint (settings->unocss->configPath), implementing a new option for VSCode, JetBrains etc. will kill DX.

This PR skips the root uno.config file when merging configs from Nuxt layers, so user should manually reexport merged config from ./.nuxt/uno.config.mjs and other tools will work as usual:

// uno.config.ts
import config from './.nuxt/uno.config.mjs'

export default config

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