Migrate MR edit page to Vue 3

What does this MR do and why?

Migrates the projects.merge_requests.edit page entrypoint to Vue 3 via @vue/compat, behind a feature flag, using the standard vue3_migration.yml flow.

Adds a beta feature flag vue3_migrate_mr_edit and a vue3_migration.yml (status: rollout) next to the entrypoint. There is an EE index.js override for this page, so an identical EE vue3_migration.yml shadow is included. No application logic changes. This page does not use Rapid Diffs.

Part of the Vue 3 migration (Code Review) epic, gitlab-org#23167.

Closes #611463

How to set up and validate locally

1. Re-run the Vue 3 infection scanner and restart the dev server:

   node scripts/frontend/infection_scanner/infection_scanner.mjs
   gdk restart rspack rails-web

2. Enable the flag: Feature.enable(:vue3_migrate_mr_edit).
3. Open a merge request's Edit page.
4. Confirm the console logs [gitlab] [V] Using Vue.js 3 (with @vue/compat) and document.querySelectorAll('[data-gitlab-vue3-app]') returns the app(s).
5. Smoke test the sidebar (labels, assignees, reviewers), the markdown editor, the target-branch selector, and saving.