feat: support snapshoting overview

[antfu]

This PR adds an experimental flag overviewSnapshots that allows to take a snapshot of a slide to display in the quick overview, to improve the performance.

[netlify]

✅ Deploy Preview for slidev ready!

Name	Link
🔨 Latest commit	179ed50
🔍 Latest deploy log	https://app.netlify.com/sites/slidev/deploys/66cd2c4cd80a980008cf2b30
😎 Deploy Preview	https://deploy-preview-1843--slidev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[pkg-pr-new]

commit: 179ed50

create-slidev

pnpm add https://pkg.pr.new/slidevjs/slidev/create-slidev@1843

@slidev/client

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/client@1843

create-slidev-theme

pnpm add https://pkg.pr.new/slidevjs/slidev/create-slidev-theme@1843

@slidev/parser

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/parser@1843

@slidev/cli

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/cli@1843

@slidev/types

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/types@1843

Open in Stackblitz