Adapt plugin for new config, improve promise resolving

[derpdead]

According to the docs plugins there is not anymore ssr option and also from the code:

// Add to nuxt plugins
    this.options.plugins.unshift({
      src: path__default['default'].join(this.options.buildDir, dst),
      // TODO: remove deprecated option in Nuxt 3
      ssr: template.ssr,
      mode: template.mode
    });

we can see that the ssr option is deprecated.

• Added support for mode instead of ssr for plugins setting.
• Promises doesn't rly work for forEach looping. Mapped plugins registration into Promise.all.