onBeforeUnmount never called

[louwers]

Describe the bug
I would expect onBeforeUnmount to be called when the next slide is shown and a component goes off screen. However, it isn't, and there is no information about the Vue lifecycle in the documentation.

To Reproduce
Steps to reproduce the behavior:

1. Create a custom component, call the onBeforeUnmount lifecycle function
2. Add it to a slide
3. Observe that it is never called

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Slidev version: 0.49.10

[louwers]

I can work around this with #1550

But it would still be good to mention that Vue's usual lifecycle hooks don't apply.