fix: [ext] in asset modules already contains a leading dot (#6826)

Author: haoqunjiang

packages/@vue/cli-service/lib/config/assets.js

@@ -5,7 +5,7 @@ module.exports = (api, options) => {
   const genAssetSubPath = dir => {
     return getAssetPath(
       options,
-      `${dir}/[name]${options.filenameHashing ? '.[hash:8]' : ''}.[ext]`
+      `${dir}/[name]${options.filenameHashing ? '.[hash:8]' : ''}[ext]`
     )
   }