Visual editor for `gitlab-ci.yml` MVC

Problem to solve

#15754 (closed) introduces a new visual builder for new .gitlab-ci.yml files. We should also apply the same functionality to a visual editing workflow.

Intended users

• Devon (DevOps Engineer)

User experience goal

Provide a visual way to edit the structured YAML

Proposal

The builder will need to be updated to allow for loading pipelines. We'll need to ensure it can't damage existing ones by losing any data.

Further details

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Is this a cross-stage feature?

Links / references