fix(types): export DatetimeHighlightStyle (#27360)

Issue number: Resolves #27353

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

`DatetimeHighlightStyle` is not automatically exported from
`@ionic/core`, since the type is not directly referenced on a prop.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Manually exports `DatetimeHighlightStyle` to consumers

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

core/src/interface.d.ts

@@ -9,7 +9,7 @@ export { ActionSheetOptions } from './components/action-sheet/action-sheet-inter
 export { BreadcrumbCustomEvent } from './components/breadcrumb/breadcrumb-interface';
 export { ScrollBaseCustomEvent, ScrollCallback, ScrollCustomEvent } from './components/content/content-interface';
 export { CheckboxCustomEvent } from './components/checkbox/checkbox-interface';
-export { DatetimeCustomEvent } from './components/datetime/datetime-interface';
+export { DatetimeCustomEvent, DatetimeHighlightStyle } from './components/datetime/datetime-interface';
 export { InfiniteScrollCustomEvent } from './components/infinite-scroll/infinite-scroll-interface';
 export { InputCustomEvent } from './components/input/input-interface';
 export { CounterFormatter } from './components/item/item-interface';