fix(ui-library): use stable references for empty object/array prop defaults

`defaultProps` created one stable instance per object/array default, but the
ES6 conversion replaced them with inline `{}` / `[]` literals that allocate a
fresh value every render. Where such a default feeds a hook dependency array
this broke referential equality and re-fired effects in a loop:

- Stepper: `steps` is in a useLayoutEffect dependency list that calls
  setStepRefs, so rendering <Stepper> with children but no `steps` looped.
- Select: `options` feeds a useMemo dependency, recomputed every render.
- TagInput: `tags` feeds a useCallback dependency, reallocated every render.

Hoist every empty object/array default to a module-level constant (matching the
old defaultProps single instance). The remaining ones (validation/labelProps/
buttonProps) are not in dependency arrays or passed to memoized children today,
but are stabilized too for parity and future safety.

Refs: Yoast/reserved-tasks#231, Yoast/reserved-tasks#246

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>