fix(admin-vite-plugin): Normalize file paths and add tests (#9595)

**What**
- #9338 had a regression which caused the import path in some virtual modules to be invalid on Windows.
- This PR fixes the issue so we now again create the correct import paths, and adds tests to prevent this from slipping in again.

packages/admin/admin-vite-plugin/package.json

@@ -22,14 +22,17 @@
   ],
   "scripts": {
     "build": "tsup",
-    "watch": "tsup --watch"
+    "watch": "tsup --watch",
+    "test": "vitest --run",
+    "test:watch": "vitest"
   },
   "devDependencies": {
     "@babel/types": "7.25.6",
     "@types/node": "^20.10.4",
     "tsup": "8.0.1",
     "typescript": "5.3.3",
-    "vite": "^5.2.11"
+    "vite": "^5.2.11",
+    "vitest": "^2.1.3"
   },
   "peerDependencies": {
     "vite": "^5.0.0"