feat(IonTabButton): publicly available el

### 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 would like `el` to be publicly available

### Describe the Use Case

To add this little blue bar, and to animate it right and left I need to get the active tab element reference, then to extract his `el`, find his position and them with a bit of math move that little bar :)

https://github.com/user-attachments/assets/52ee9af0-ddc3-43ac-a21b-6c7c5fca1489

### Describe Preferred Solution

Make the `el` being publicly available

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

I'd be more than happy to do a PR if this issue gets approved