Option to turn off PWA generation

What problem does this feature solve?

If building for multiple targets, some of them might not support PWA. It would be great to have an option to turn off the functionality

What does the proposed API look like?

Currently the only way is to use a env var VUE_CLI_BUILD_TARGET different to app: https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-plugin-pwa/index.js#L4

This line could e.g. be changed to:

if (target && target !== 'app' || options.pwa.off === true) {
      return
}

[zzx0106]

config.plugins.delete(‘pwa’); config.plugins.delete(‘workbox’);

[LinusBorg]

I think this makes sense. I’ll do make a PR.