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.

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[pkg-pr-new[bot]]

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

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