Improve devfile editing UX

Is your enhancement related to a problem? Please describe

To simplify the day 1 activities (start a sample workspace, start a workspace from a git repository) we have removed the Devfile editor from the Dashboard as editing a YAML file to create a workspace was a daunting activity for new users.

As a consequence we have sacrificed a day 2 activity, something that more experienced users loved: the devfile editor. From devfile editor in the dashboard allowed to create and edit a devfile (re)start a workspace from it.

Describe the solution you'd like

There will be multiple alternatives:

The progressive editing flow is the following one:

  1. start a workspace without Devfile: empty or from a git repository (passed as a URL parameter)
  2. in the IDE it will be possible to use a GUI tool to generate or to edit the devfile (no YAML)
  3. from the IDE it will be possible to restart a workspace from the created / modified devfile
  4. for advanced configuration it will be possible to edit a Devfile YAML with code completion and API documentation in the IDE itself

Subtasks:

Describe alternatives you've considered

We want to avoid the re-introduction of the user dashboard devfile editor because we want that to happen in the IDE. That's where a developer has the best development experience and that's where they expect to edit their workspace. Having to go back to the dashboard to edit the workspace is awkward. This is a feedback we got recurrently from users.

Additional context

https://issues.redhat.com/browse/CRW-3152