fix: set default symlinks to true

Currently in Vue CLI we set the default `symlinks` config to `false`.
This setting is counterintuitive, and inconsitent with webpack's default
configuration, per https://webpack.js.org/configuration/resolve/#resolve-symlinks

The change of default `symlinks` value is introduced by vuejs@5b4df14
It is most likely a duplicate of vuejs@145492b

Since several third-party npm clients uses symlinks to speed up installation
(e.g. cnpm, pnpm...), that change breaks webpack HMR for their users.
See vuejs#1559, vuejs#2195, vuejs#2284

This commit also fixes vuejs#1609

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

@@ -37,7 +37,6 @@ module.exports = (api, options) => {
         .publicPath(options.baseUrl)
 
     webpackConfig.resolve
-      .set('symlinks', false)
       .extensions
         .merge(['.js', '.jsx', '.vue', '.json'])
         .end()