feature: Add vuepress2 plugin PWA support

[Zhengqbbb]

Name:

• vuepress2 official plugin pwa and pwa pop

  Advantage:

• Good interactive experience Progressive Web Apps

• Works well with document content updates in the browser's cache

[Zhengqbbb]

Tip: If you want to test the PWA, you can use anywhere.

[Renovamen]

Closed by #32.