[EuiSelectableList] Migrate from class to function component

`EuiSelectableList` renders the virtualized, accessible options list within `EuiSelectable`. Performance-sensitive due to potentially large option sets.

- **File:** `components/selectable/selectable_list/selectable_list.tsx`
- **Class size:** 443 lines
- **State:** `defaultOptionWidth`, `optionArray`, `itemData`, `ariaPosInSetMap`, `ariaSetSize`
- **Lifecycle:** `shouldComponentUpdate`, `componentDidUpdate`, `componentWillUnmount`

**Migration notes:**

Uses `shouldComponentUpdate` for performance optimization — must be replaced with `React.memo` and careful memoization. Complex ARIA attribute computation. Should be migrated with `EuiSelectable`.

**Acceptance criteria:**

- [ ] Convert to function component wrapped in `React.memo`
- [ ] Replace `shouldComponentUpdate` with `React.memo` comparison
- [ ] Maintain virtualization and ARIA attributes
- [ ] Performance benchmark before/after migration
- [ ] All existing tests pass