[tools/version]: create a new version from a spec already populated by generators

[fedeci]

Currently, because we are using JSON.parse to print the spec, generators and structures that are not part of JSON do not get printed correctly.

Errors found when transforming fig spec:
Before:

const generator =  {
  script: 'foo',
  postProcess: () => {}
}

// ... spec here
args: {
  generators: generator
}

After:

// ... spec here
args: {
  generators: { script: 'foo' }
}

---

const generator =  {
  custom: () => {}
}

// ... spec here
args: {
  generators: generator
}

After:

// ... spec here
args: {
}