feat(checkbox): add checkbox missing focus on tab (#30075)

Issue number: internal

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

- Added ion-focusable class to ion-checkbox
- Replaced ionic focus styles for focus mixin

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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

- [Ionic states
sample](https://ionic-framework-my9hzgra2-ionic1.vercel.app/src/components/checkbox/test/states?ionic:theme=ionic)

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Co-authored-by: ionitron <hi@ionicframework.com>