feat: allow adding Vite plugins dependent on slides data

[jgosmann]

This would enable some advanced uses cases for me. I couldn't find any other method to access slide data in my Vite plugin so far.

[netlify]

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[pkg-pr-new]

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create-slidev

npm i https://pkg.pr.new/slidevjs/slidev/create-slidev@2112

@slidev/client

npm i https://pkg.pr.new/slidevjs/slidev/@slidev/client@2112

create-slidev-theme

npm i https://pkg.pr.new/slidevjs/slidev/create-slidev-theme@2112

@slidev/parser

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@slidev/cli

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@slidev/types

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commit: 35fe871