While resolving JENKINS-47752 I noticed there were some flow errors in the JDL, as it's not being run as part of CI but probably should.

• Enable flow in all project using it (see below). This should probably be added to "mvnbuild" script, basically run at the same time as eslint since it performs similar function.
  • jenkins-design-language
  • blueocean-core-js
  • blueocean-pipeline-editor
• Resolve any flow errors in above projects so build passes
  • NOTE that the scope of this might be large, so we may need to pragmatic in the approach if it becomes a massive effort