feat(nuxt3): extends support for plugins/ directory

[kevinmarrec]

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Following up nuxt/nuxt#13367 (extends support progress) this PR allows extending plugins/ directory for extended config layers

TO DO

• Merge feat!(nuxt3): extends support for composables/ directory #3423 beforehand so we get the srcDir thing for layers
• Extends support for plugins directory
• Extends support for plugins config key in layers => Need to resolve aliases

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[pi0]

For supporting aliases in layer, we might introduce new named layer aliases. Changing resolver behavior and aliases might be tricky to be handled properly and sometimes we actualyl do like to allow overriding something from user dir (~/plugins/foo being overriden)

export default defineNuxtConfig({
  name: 'myTheme'
  plugins: [
    '~myTheme/plugins/foo'
  ]
})