Auto-fill

Give the builder a list of prefillSources and every field gets an Auto-fill from… choice (stored as field.config.prefill, with config.prefillLock to make the value read-only). Texts can use {{key}} tokens, e.g. a description of {{contact.address}}. When the form is opened, the host calls applyPrefill(document, data) from @restrella/form-generator and renders the returned doc with the returned values as the generator's model.