feat(nuxt): experimental flag to use external vue dependencies

[danielroe]

🔗 Linked issue

nuxt/nuxt#13632

❓ 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

This PR starts exploring implementing moving vue, @vue/* and vue-router to externals. As it is, it is not the solution. Although it works, we have (at least) the following issues:

• multiple copies of imports are included, for almost all vue and @vue libs, meaning an increase of ~2.2M (688kB -> 2.9M) in node_modules for a minimal project using pages/ - but note that the total.output/server size decreases slightly from 3.3M -> 3.2M. (Note: we can improve this into a much larger decrease by implementing import conditions upstream in vue.)
• there are extra, non-treeshaken bits of vue-router that we could prune (such as other forms of router history and all code behind isBrowser flag)
• unneeded parts of the vue api, such as @vue/devtools-api, are included in server node_modules

Some ideas to explore:

• 'light' processing of Vue externals - extra compilation steps to strip browser or dev-only code?
• manual post-build step to remove dev cjs builds
• import conditions (development/production) for Vue core libraries

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[pi0]

@danielroe While this should be default soon, can we please have the externalization behind an experimental flag in order to merge and test earlier? 🙏🏼 I belive this can also help to try and find nitro enhancements to tree-shake node_modules