fix(angular): add old IonBackButtonDelegate import (#28831)

Issue number: resolves #28827

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## What is the current behavior?
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This import was renamed to `IonBackButton` as part of
9f20780.
However, we did not consider that some developers are importing this to
use with `ViewChild`. While we should keep alignment with other
component imports, we need to support the old import until we can make a
breaking change in Ionic 8.

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

- Adds the old IonBackButtonDelegate import

## 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. -->
Dev build: `7.6.5-dev.11705352188.1c6cdb08`

packages/angular/src/index.ts

@@ -6,6 +6,8 @@ export { SelectValueAccessorDirective as SelectValueAccessor } from './directive
 export { TextValueAccessorDirective as TextValueAccessor } from './directives/control-value-accessors/text-value-accessor';
 export { IonTabs } from './directives/navigation/ion-tabs';
 export { IonBackButton } from './directives/navigation/ion-back-button';
+// TODO FW-5889
+export { IonBackButton as IonBackButtonDelegate } from './directives/navigation/ion-back-button';
 export { IonNav } from './directives/navigation/ion-nav';
 export { IonRouterOutlet } from './directives/navigation/ion-router-outlet';
 export {