Vuepack 4.0 progress

[shanoaice]

Vuepack had not been actively maintained for a long time, and everything had changed a lot during this period. This is an issue to keep track of works toward Vuepack 4.0.

• Upgrade the saofile to SAO v1 format
• Rewrite the Webpack config almost from scratch using webpack-chain
• Upgrade Electron to the latest version
• Abandon Vue CLI (on 4.0.0 stable)