fix: import vue.config.mjs on windows

Author: Staremang

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

@@ -1,5 +1,6 @@
 const fs = require('fs')
 const path = require('path')
+const URL = require('url')
 
 const isFileEsm = require('is-file-esm')
 const { loadModule } = require('@vue/cli-shared-utils')
@@ -25,7 +26,7 @@ module.exports = function loadFileConfig (context) {
     const { esm } = isFileEsm.sync(fileConfigPath)
 
     if (esm) {
-      fileConfig = import(fileConfigPath)
+      fileConfig = import(URL.pathToFileURL(fileConfigPath).href)
     } else {
       fileConfig = loadModule(fileConfigPath, context)
     }