optimize build time by custom terser config and little faster ts

[atomfrede]

This pr improves the production build time. On my machine in the end it was ~8s faster (not much). What does it do?

• changed config for ts loader (type checking via https://github.com/TypeStrong/fork-ts-checker-webpack-plugin which is doing it async). The config is in line with the one used by vue cli
• align the terser plugin with vue cli
• regular dependency update (does not have impact on build times)

I also tried a hello world app created with vue-cli and the production build (for basically one page) takes already 4.5s on my machine, so having for a full blown app like ours taking ~16 - 17s on my machine looks at least "ok"

Not doing sourcemaps with terser plugin cuts again 3-4s from the overall build time.

updates #620

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[pascalgrimaud]

thanks @atomfrede :)