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>

core/src/components/checkbox/checkbox.ionic.scss

@@ -380,8 +380,7 @@ input {
 // --------------------------------------------------
 // Only show the focus ring when the checkbox is focused and not disabled
 :host(.ion-focused:not(.checkbox-disabled)) .native-wrapper {
-  outline: var(--focus-ring-width) globals.$ion-border-style-solid var(--focus-ring-color);
+  @include globals.focused-state(var(--focus-ring-width), globals.$ion-border-style-solid, var(--focus-ring-color));
-  outline-offset: var(--focus-ring-offset);
 }
 
 // Checkbox: Active

core/src/components/checkbox/checkbox.tsx

@@ -212,6 +212,7 @@ export class Checkbox implements ComponentInterface {
           'in-item': hostContext('ion-item', el),
           'checkbox-checked': checked,
           'checkbox-disabled': disabled,
+          'ion-focusable': true,
           'checkbox-indeterminate': indeterminate,
           interactive: true,
           [`checkbox-justify-${justify}`]: justify !== undefined,