Option to emit TS declaration files when build target is Library

[Robula]

What problem does this feature solve?

Whilst we can build component libraries using --target lib, if these components are built and consumed in Typescript projects they are missing their types.

A workaround is to modify the webpack configuration but I'm sure this is a common use case to warrant this being either the default behavior or at the very least having an option to enable it.

chainWebpack: config => {
    config.module
      .rule('ts')
      .uses
      .delete('thread-loader')

    config.module
      .rule('ts')
      .use('ts-loader')
      .tap(options => {
        options.transpileOnly = false
        options.happyPackMode = false
        options.compilerOptions = {
          declaration: true,
          noEmit: false,
          outDir: 'lib'
        }
        return options
      })
}

What does the proposed API look like?

N/A

[Robula]

Seems like this duplicates #1081 however I am on 4.5.13, not v3.

[mater1996]

use vue-dts-gen ?