Add ?vue3 infection plugin for iterative Vue 3 migration in Vite

What does this MR do and why?

Introduces a Vite plugin that enables per-entrypoint iterative Vue 3 migration. By appending ?vue3 to a JS import specifier, the entire dependency subtree becomes "infected" — Vue ecosystem packages (vue, vuex, vue-router, vue-apollo, etc.) are redirected to their Vue 3 compat wrappers, while non-Vue imports get separate module instances to ensure isolation.

This allows migrating individual page entrypoints to Vue 3 one at a time without changing the global VUE_VERSION environment variable, enabling gradual adoption.

How to set up and validate locally

1. Ensure you have a working GDK with Vite enabled
2. Add ?vue3 to any JS entrypoint import, e.g.:

   import initFeature from 'ee/some_feature/bundle?vue3';

3. Run gdk start and navigate to the page using that entrypoint
4. Open browser DevTools Network tab — infected modules should have ?vue3= in their URLs
5. Vue ecosystem imports (vue, vuex, etc.) should resolve to vue3compat/ wrappers

Known issues:

• vite production build (will be addressed in follow-up MR)
• this is as dumb as possible - it will duplicate every possible module it discovers. this is intentional
• HMR. It might or might not work 🌚

MR acceptance checklist

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