Support `category` meta-data in element template

[nikku]

Is your feature request related to a problem? Please describe

At some point we want to support grouping of element templates by category. To do this users shall be able to specify that meta-data via the element template descriptor (cf. .

Describe the solution you'd like

Be able to specify { name, id } as category with an element template descriptor:

{
  "id": "my-special-template",
  ...,
  "category": {
    "name": "Super special templates",
    "id": "super-special-templates"
  }
}

This allows us to group templates in places where they are shown as a batch, cf. bpmn-io/element-template-chooser#5 (comment).

• Add + validate category in JSON Schema

Describe alternatives you've considered

None.

Additional context

Example of a grouping:

[nikku]

This is already supported in our modeling tooling; just needs proper schema support.