refactor!: Use JSON instead of XML for defining dynamic toolbox categories.

[gonfunko]

The basics

• I validated my changes

The details

Resolves

Fixes #5580

Proposed Changes

This PR updates the variable, dynamic variable, and procedure toolbox category definitions to use the newer JSON schema in place of the older XML-based format.

Reason for Changes

Support for XML will eventually be removed, and this serves as both eating our own dogfood and a practical example of implementing a dynamic category definition with JSON which third party developers may benefit from as a reference.

Deprecations

• The Element[] return type of the flyoutCategory() method for each category is now deprecated. Users can opt-in to the new FlyoutItemInfo[] (JSON) return type by passing a second argument of false to the method. We expect to switch the default return type in v13.

[rachel-fenichel]

Since we are not deleting the XML code yet, we should keep the old XML functions around.

They are exported, and external devs may be calling them from their own custom flyout categories. I think that means we need to add new functions to do all of the JSON work you're doing here, while marking the old functions as deprecated either in cod eor in comments.