switch webpack to development mode when use "--debug"

[huww98]

Hi all, today I spent hours trying to debug the client-side error "Hydration completed but contains mismatches". To view the warnings from vue, I ended up adding the following to my config:

  bundlerConfig: {
    configureWebpack(config, isServer, isBuild) {
      return { mode: 'development' };
    }
  },

How about we can simplify this by enabling the development mode by default when invoked as yarn build --debug

BTW, the mismatched element is the "hash" field from @vuepress/plugin-debug, maybe that component should be ClientOnly to avoid the error?

[Mister-Hope]

Track #734