fix: windows vue.config.mjs support (#7023)

packages/@vue/cli-service/lib/util/loadFileConfig.js

@@ -1,6 +1,6 @@
 const fs = require('fs')
 const path = require('path')
-
+const { pathToFileURL } = require('url')
 const isFileEsm = require('is-file-esm')
 const { loadModule } = require('@vue/cli-shared-utils')
 
@@ -25,7 +25,7 @@ module.exports = function loadFileConfig (context) {
     const { esm } = isFileEsm.sync(fileConfigPath)
 
     if (esm) {
-      fileConfig = import(fileConfigPath)
+      fileConfig = import(pathToFileURL(fileConfigPath))
     } else {
       fileConfig = loadModule(fileConfigPath, context)
     }