Have develop command pick next randomly available port for dev and live reload servers

Type of Change

• New Feature Request
• Documentation / Website
• Improvement / Suggestion
• Bug
• Other (please clarify below)

Summary

By default, instead of hardcoding the port for devServer it might be really nice to have it just pick the next available port on the user's local machine? This would in particular support running multiple instances of the dev server locally.

In addition, we should make sure to support this for the livereload port, since that will break in a similar use case (running multiple greenwood develop instances)

Details

Something similar to webpack-dev-server

Also, if we enable this, we might / should be able to run all our tests in parallel? Doesn't have to be in this PR, but a good fast follow if so! If we do run our tests in parallel, just need to be aware of this case