Make --stdin flag of webpack available by vue-cli-service

[LostKobrakai]

What problem does this feature solve?

Same rational which lead webpack to implement the flag in the first place: webpack/webpack#1308

What does the proposed API look like?

vue-cli-service serve|build --stdin or vue-cli-service serve|build --watch-stdin

[haoqunjiang]

The --stdin flag is added in 4.2.