feat(app): update webpack-dev-server to v4.0.0-rc.0

[hawkeye64]

The only issue I saw is handling for this which may need extra tweaking:

• host and port options can't be null or empty string
  https://github.com/webpack/webpack-dev-server/releases/tag/v4.0.0-rc.0

[LeoniePhiline]

Yes! This update is dearly needed.

Right now, neither the old nor the new option validate successfully:

Old:

  devServer: {
    sockPort: 443,
  },

(This fails, because sockPort no longer validates.)

New:

  devServer: {
    webSocketURL: {
      port: 443,
    },
  },

(This fails, because webSocketURL does not validate in webpack-dev-server@4-beta, but that is fixed in rc.0.)

See also: https://github.com/webpack/webpack-dev-server/blob/master/migration-v4.md

[LeoniePhiline]

@hawkeye64 Why did you close the PR?

[therealcoder1337]

@LeoniePhiline i guess because more work is needed, as i outlined in #10083 (comment) and #10083 (comment)
simply bumping the dependency will not help, because of needed migration steps.

[LeoniePhiline]

Ah I see. Thank you!