Add webpack Vue 3 infection plugin for iterative migration

What does this MR do and why?

Implements the webpack equivalent of the Vite Vue 3 infection plugin to enable per-entrypoint Vue 3 migration in webpack builds. When a JS file is imported with ?vue3, the plugin infects the entire dependency subtree — Vue ecosystem packages are redirected to their Vue 3 compat wrappers via CONTEXT_ALIASES while non-Vue imports get separate module instances through ?vue3 resource query appending.

Key changes

• config/plugins/webpack_vue3_infection_plugin.js (new): Webpack 4 plugin using normalModuleFactory beforeResolve/afterResolve hooks with WeakSet-based dependency tracking for reliable infection propagation across the resolve pipeline
• config/helpers/context_aliases_shared.js: Extract INFECTABLE_RE and INFECTION_BLOCKLIST from the Vite plugin so both plugins share identical infection rules
• config/helpers/vite_plugin_vue3_infection.mjs: Import shared constants instead of defining them locally
• config/webpack.config.js: Enable plugin conditionally when !USE_VUE3, matching the Vite plugin's activation condition

How infection works in webpack

1. beforeResolve: Detects ?vue3 query on import requests (infection roots) or checks if the importer is already infected via a tracked Set. Rewrites Vue ecosystem imports to CONTEXT_ALIASES targets. Skips inline-loader requests (containing !) and vue-loader sub-requests (?vue&type=...) to avoid infecting webpack/loader internals.
2. afterResolve: For infected dependencies, appends ?vue3 to the resolved resource, creating separate webpack module instances (webpack deduplicates by request which includes the query string).
3. finishModules: Reports infection stats (total, infected, duplicated module counts).

How to set up and validate locally

1. Add ?vue3 infection to an entrypoint:

--- a/app/assets/javascripts/pages/groups/work_items/index.js
+++ b/app/assets/javascripts/pages/groups/work_items/index.js
 import { NAMESPACE_GROUP } from '~/issues/constants';
-import { initWorkItemsRoot } from '~/work_items';
+import { initWorkItemsRoot } from '~/work_items?vue3';

2. Ensure gdk stop vite (falls back to webpack)
3. Open the page — check console for [vue3-infection] stats showing infected module count > 0
4. Verify no Critical dependency warnings related to the infection

MR acceptance checklist

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