feature request: schema variable support

as I'm creating Shopify blocks and there is a limit on the app block size, list settings become very cumbersome due to their size:

    {
      "type": "select",
      "id": "one",
      "label": "One",
      "options": [
        { "value": "a", "label": "A" },
        { "value": "b", "label": "B" },

if I would create 10 drop downs with A-Z selectors, they would almost hit the limit. It would be nice to create something like:

{% schema %}
{%- assign options = [...-%}
...
    {
      "type": "select",
      "id": "one",
      "label": "One",
      "options": options

a scoped variable to the schema or just somehow create:

{% schema %}
{ 
 "variables": { "options": [] }

and reference from inside the schema (probably easier if schema is a json, just search for that value inside the schema under "variables"

Start by reviewing the repository's handling of the {% schema %} block and the variable approaches described in the issue. Done would mean a defined, tested way to reuse option data inside schema JSON without duplicating large lists or exceeding the app block size limit.