Default OGP Setting

[ogadra]

Related #1932

Is your feature request related to a problem? Please describe.

If #2096 is merged, we will be able to use OGP setting on Slidev pages.

On Social media, slides are often shared, and the first slide is typically used as the OGP image.

Therefore, I would be even happier if an image of ther first slide could be automatically generated and set as the OGP image when building a Slidev project.

Describe the solution you'd like

When the project is built, generate a PNG image of first slide and set it as the OGP image for the page.

Describe alternatives you've considered

I have no idea.

[ogadra]

I checked the source code. It looks like we need to implement a way to specify which slide to export via an argument, because currently, all slides are exported instead of just the first one.