TypeScript: have items and selectedItems as Readonly

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- `npm` (or `yarn`) version:

**Relevant code or config**

```javascript
```

**What you did**:

**What happened**:

**Reproduction repository**:

**Problem description**:
Since we use items/selectedItems just for some logic and we do not tamper with them, we can mark them as read only in the TS interface.

**Suggested solution**:
In typings.ts for useSelect, useCombobox and useMultipleSelection, use Readonly to wrap the items type.