feat: add useDuration helper

[Tandashi]

What has changed?

This Pull Request adds a new helper / utility function that allows for easier creation and usage of TimeEvents for animations.

Furthermore, the documentation has been extended and now features a subsection ("Controlling animation duration with Time Events") under the Time Events chapter.

Closes: #171

---

Example

Let's look at the following example where we want to animate the scale of a circle:

import {makeScene2D} from '@motion-canvas/2d';
import {Circle} from '@motion-canvas/2d/lib/components';
import {createRef, useScene} from '@motion-canvas/core/lib/utils';

export default makeScene2D(function* (view) {
  const circle = createRef<Circle>();

  view.add(
    <Circle ref={circle} width={320} height={320} fill={'lightseagreen'} />,
  );

  useScene().timeEvents.register('event');
  const duration = useScene().timeEvents.get('event').offset;
  yield* circle().scale(2, duration);
});

In the example a TimeEvent called event is registered and its duration gets used to control
the duration of the circle's scale animation.

With the new useDuration function, this can be reduced to a single line of code as follows:

yield* circle().scale(2, useDuration('event'));

[aarthificial]

This is an internal implementation detail, I don't think we should include it in the docs.
I'd simply say:

Aside from specifying _when_ something should happen, time events can be used to 
control _how long_ something should last. You can use the 
[`useDuration`](/api/core/utils#useDuration) function to retrieve the duration of an event
and use it in your animation:

```ts
yield* circle().scale(2, useDuration('event'));
```

[aarthificial]

The entire section is about time events so I think it could be just:

Suggested change

	## Controlling animation duration with Time Events
	## Controlling animation duration

[aarthificial]

Thanks for the PR!