feat(select): add <optgroup> (group wise) list and the ability to search in the modal interface

### Prerequisites

- [x] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/docs/CONTRIBUTING.md#creating-an-issue).
- [x] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [x] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already include this feature request, without success.

### Describe the Feature Request

I need to add and also the option to do search using with a modal interface. Below is the example link.

[https://www.w3schools.com/tags/tag_optgroup.asp](https://www.w3schools.com/tags/tag_optgroup.asp)

[https://codepen.io/rjleurck/pen/PxZKYr](https://codepen.io/rjleurck/pen/PxZKYr)

[https://codepen.io/cbrewster/pen/PKKQyJ](https://codepen.io/cbrewster/pen/PKKQyJ)

### Describe the Use Case

I need to add and also the option to do search using with a modal interface. Below is the example link.

[https://www.w3schools.com/tags/tag_optgroup.asp](https://www.w3schools.com/tags/tag_optgroup.asp)

[https://codepen.io/rjleurck/pen/PxZKYr](https://codepen.io/rjleurck/pen/PxZKYr)

[https://codepen.io/cbrewster/pen/PKKQyJ](https://codepen.io/cbrewster/pen/PKKQyJ)

### Describe Preferred Solution

_No response_

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

Start by reading the ion-select component and its modal interface; the issue names no implementation files or tests. Clarify the expected optgroup structure, modal search behavior, and acceptance criteria before identifying the relevant component tests.

索引モデルが issue の本文から書いたものです。