Upgrade vuepress to v2 and add dark mode support

[hustcer]

Upgrade vuepress to v2 and add dark mode support

This might be a long journey, so I will create a draft first.

Check List

• Make sure all node dependencies upgrade to the latest version, especially vuepress related modules;
• Make sure dark mode works;
• Make sure all the styles work correctly as before;
• Make sure all the plugins work well;
• Make sure every page / table rendered successfully;

Anything else need to be checked?

[rgwood]

Thank you so much for this! Amazing work!

[stormasm]

wow ! super fantastic 😃 great job....