feat(config): transforms for index.html

[antfu]

This PR introduced a new config indexHtmlTransforms which allows plugins and vite.config.js to transform the index.html for various needs. (not sure about the naming though, feel free to leave comments)

For example, it would be useful for these plugins(probably not exist yet, just giving the context

• vite-plugin-meta
• vite-plugin-sitemap
• vite-plugin-pwa
• vite-plugin-minify
• etc.

the following config replaces page's title to Vite Playground

// vite.config.js

export default {
  indexHtmlTransforms: [
    {
      flush: 'pre', // or 'post'
      transform({ code, isBuild }) {
        return code.replace(/<title>.*?<\/title>/, '<title>Vite Playground</title>')
      }
    }
  ]
}

flush also provides the ability to control when should the transforms apply (before Vite's processing or after)

Resolves #528, #739

Related #639, vuejs/vitepress#77 (comment)

[antfu]

@knightly-bot build this

[knightly-bot]

🌒 Knightly build enabled, release every night at 00:00 UTC ^{(skip if no change)}

📦 npm package

[yyx990803]

Made some small adjustments:

• flush -> apply
• Defaults to apply: 'post', so the transform can directly be a function instead of always having to use an object with explicit apply timing.