fix: rename validationOptions to legacyOptions

[sandrina-p]

Follow-up of #211, comment:

note: Now looking back I'd have called this v0Support instead of ValidationOptions. This is where we'd also pass any config to still support deprecated features.

Another one that comes to my mind is dropping type attr in favor of of inputType. If we have done that already, then it would look like this:

const form = createHeadlessForm(schema, {
  v0Support: {
      includeTypeAttr: true,
  }
}

[antoniocapelo]

@sandrina-p I'm a bit uncertain about the v0Support name… let's imagine we want to keep this option on a future version. Imagine we introduce a breaking change next week, and we'll be on v2 — having v0Support in it will look off.

Can we find a better name? Maybe Options + the new JSDoc comments you added mentioning v0 is enough?

[antoniocapelo]

LGTM!