Repository: angular
Issue: Create or define type for animation transition input
Labels: area: animations

### Describe the problem that you experienced

When you create a new animation that needs dynamic or calculated inputs, you'd do so using the syntax like so: 

```html
<div [@animationName]="{ value: 'stateName', params: buildAnimationParams() }"></div>
```

I can only find this syntax defined in once place which is on the API docs page for the [transition](https://angular.io/api/animations/transition) function.

Poking through the source code, I found where I believe this input is used in `transition_animation_engine.ts` with the class `StateValue` ([link](https://github.com/angular/angular/blob/main/packages/animations/browser/src/render/transition_animation_engine.ts#L106)). In that source file, `value` is typed in the constructor code as `any` (as seen with the input and return types of the `normalizeTriggerValue` function). In the field declarations of the class however, it is typed as a `string`. Likewise the `input` property for the constructor is typed as `any`.

### Enter the URL of the topic with the problem

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### Describe what you were looking for in the documentation

When looking through the documentation, I was looking for what inputs are accepted for the animation trigger binding. All I could find was this excerpt from the Introduction page:

> Then, you can bind the trigger to a template expression using standard Angular property binding syntax as shown below, where triggerName is the name of the trigger, and expression evaluates to a defined animation state.

### Describe the actions that led you to experience the problem

In my project, I have on the lint rule that requires all functions to have a defined return type. That proved difficult when I was trying to define functions that return the state name and animation params like so:

```typescript
@HostBinding('@rotateIcon')
get rotateIconBinding() {
  return this.rotateDegrees === 0 ? this.getDefaultAnimationState : this.getRotatedAnimationState;
}

protected get getDefaultAnimationState() {
  return { value: 'default', params: { rotateDegrees: this.rotateDegrees } };
}

protected get getRotatedAnimationState() {
  return { value: 'rotated', params: { rotateDegrees: this.rotateDegrees } };
}
```

### Describe what you want to experience that would fix the problem

Any (preferably all) of the following would help alleviate the issue for me or others that are struggling with the state of the current animation docs:

1. Create a type definition for the `input` property of `StateValue` like so:
  ```typescript
  import { AnimationOptions } from '@angular/animations';

  export type AnimationStateInput = string | ({ value: string } & AnimationOptions);
  ```
2. Add a section in the Introduction docs explaining what inputs can be provided (aka the name or an object defined like step 1)

The above solutions would fix the documentation problem, but it would also be fantastic if there was some sort of IDE support that could detect available animation states so that when you work with animations, it detects issues at compile time.

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