#3599016 Autocomplete elements

Expose the webform_autocomplete and entity_autocomplete element types.

webform_autocomplete gains its static settings (autocompleteItems, autocompleteLimit, autocompleteMatch, autocompleteMatchOperator), backed by the WebformAutocompleteMatchOperator enum. autocompleteItems resolves the configured suggestions to a flat, deduped list of label strings.

entity_autocomplete gains a defaultEntities field resolving its pre-selected entities to {id, label}, plus a webformEntityAutocomplete Query that returns live matches through the element's configured selection handler.

Closes #3599016