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/tsup.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from "tsup"
+
+export default defineConfig({
+  entry: ["./src/index.ts"],
+  tsconfig: "tsconfig.build.json",
+  format: ["cjs", "esm"],
+  dts: true,
+  clean: true,
+})