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/src/custom-fields/generate-custom-field-forms.ts

@@ -29,7 +29,7 @@ import {
   traverse,
 } from "../babel"
 import { logger } from "../logger"
-import { crawl, getParserOptions } from "../utils"
+import { crawl, getParserOptions, normalizePath } from "../utils"
 import { getConfigArgument, getModel, validateLink } from "./helpers"
 
 type CustomFieldConfigField = {
@@ -263,7 +263,8 @@ function generateCustomFieldConfigName(index: number): string {
 }
 
 function generateImport(file: string, index: number): string {
-  return `import ${generateCustomFieldConfigName(index)} from "${file}"`
+  const path = normalizePath(file)
+  return `import ${generateCustomFieldConfigName(index)} from "${path}"`
 }
 
 function getForms(