Prompt Megastep

This is a rough sketch of how we'll consolidate all existing generative steps (Make Text, Make Image, etc.) into a single "Prompt" step.

Currently, these are all distinct steps with their own types:

• a2.bgl.json#[name]
• go-over-list.bgl.json
• etc.

The new "Prompt" step will combine them into one new step, with its own new type:

• prompt.bgl.json

The new step will:

• import a2.bgl.json as a framework
• have an extra generative-mode configuration port that will allow selecting various generative modes (aka steps). The values of the generative-mode configuration port will map to the previously distinct steps:
  • "Generate Text" -> "Make Text" -> a2.bgl.json#daf082ca-c1aa-4aff-b2c8-abeb984ab66c
  • "Generate Image" -> "Make Image" -> ...
  • ...
• while being edited, use the "Reactive" port capability (landed in #5259, see reactive-step.ts for an example) and switch the rest of configuration dynamically based on the mode, calling describe on the matching distinct step and inserting its properties inline.
• when run, will read the value of the generative-mode port and call invoke on the matching distinct step

Mapping old steps to new steps

In BGL, the new Prompt step will have a new prompt.bgl.json type, and in configuration, the generative-mode property that will contain the old distinct step URL (filed #5260 to implement).