feat: add CSS variables or shadow part for customizing toggle on/off label color

### Prerequisites

- [X] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/.github/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

The toggle component in `ios` currently has no way to style the on/off label colors except for by customizing `--ion-color-dark` or `--ion-color-contrast`. See the following code:

https://github.com/ionic-team/ionic-framework/blob/597bec753412758516e69725a89f57970fbb6f6e/core/src/components/toggle/toggle.ios.scss#L90
https://github.com/ionic-team/ionic-framework/blob/597bec753412758516e69725a89f57970fbb6f6e/core/src/components/toggle/toggle.ios.scss#L108

I think we should avoid having users customize the Ionic colors on a per-component basis.

### Describe the Use Case

If you customize the toggle by using the following:

```html

```

```css
ion-toggle {
--background: #000;
--background-checked: #f3e895;

--handle-background: #f3e895;
--handle-background-checked: #000;
}
```

You are not able to see the on/off labels in light mode. Here is a demo: https://stackblitz.com/edit/suseoe-jzo3g2?file=index.html

### Describe Preferred Solution

I believe we should either add CSS variables for the color & color-checked state or we should expose it as a shadow part which would also allow users to customize the size of the labels. As it is right now they can't change the `width` or `height` either.

### Describe Alternatives

_No response_

### Related Code

https://github.com/ionic-team/ionic-framework/blob/main/core/src/components/toggle/toggle.ios.scss#L58-L118

### Additional Information

_No response_

Start with core/src/components/toggle/toggle.ios.scss, especially the referenced lines and the broader range around lines 58-118. Review how the iOS toggle renders its on/off labels and determine whether CSS variables or a shadow part best exposes their color and sizing. Done means users can customize label color and dimensions without overriding global Ionic colors.

索引モデルが issue の本文から書いたものです。