Support specialization derivation type

[aartaka]

Before ce681c1, specialization was supported, but now it throws Can't recognize element type 'http://hl7.org/cda/stds/core/StructureDefinition/ON' (specialization).

Config to reproduce:

import { defineConfig } from '@atomic-ehr/codegen';

export default defineConfig({
  packages: ['hl7.cda.uv.core@2.0.1-sd'],
  typescript: {
    generateIndex: true
  },
  outputDir: "./hl7-cda-uv-core"
});