Build out the `form-js-viewer-core` package

[Skaiir]

Is your feature request related to a problem? Please describe.

As part of the current refactoring project to separate the form-viewer into a renderer and core part. This issue tracks the core side.

Describe the solution you'd like

Our goal is to create a library which is basically a large transformation pipeline. It manages the form state, and applies multiple transformations based on the current state, the schema, and the initial state.

Breakdown

We will follow the pipeline outlined above to have a progression. There may be some missing aspects here, but this is a good level to break things down at

• [ ] Schema importer API
• [ ] State management API
• [ ] Model "render" pipeline
  • [ ] Milestone 1 - Naive render step - Date TBA
    • [ ] Expands out dynamic lists from the state
    • [ ] Provide a model that the viewer can render (giant JSON object)
  • [ ] Milestone 2 - Top down visibility calculation - Date TBA
    • [ ] Manages a "working state" which removes elements that are hidden
    • [ ] Visibility is computed from top to bottom
    • [ ] Provides the visibility adjusted state easily
    • [ ] Provides the visibility adjusted form model easily
  • [ ] Milestone 3 - Feel evaluation layer (except visibility) - Date TBA
    • [ ] Uses the visibility adjusted state + initial state to compute feel & feelers across the form schema
    • [ ] Provides the visibility adjusted and feel computed form model easily
  • [ ] Milestone 4 - Pipeline extension points - Date TBA
    • [ ] Pre-feel model enrichment
    • [ ] Post-feel model enrichment
      • [ ] Validation computation
      • [ ] Metadata / API for viewer
• [ ] Alignment check with the core module
• [ ] (to detail) performance & memoization
• [ ] Test migration

Describe alternatives you've considered

n/a

Additional context

Root issue: https://github.com/bpmn-io/form-js/issues/1166

[marcosgvieira]

@Skaiir as agreed, let's break the milestones here so it's easier to understand when we target to deliver.