fix(types): export DatetimeHighlightStyle (#27360)

Issue number: Resolves #27353

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

## What is the new behavior?
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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. -->