feat(html): add html.additionalAssetSources option

[monam2]

Issue

close #21182

Description

This PR adds the html.additionalAssetSources option to allow users to configure custom HTML elements and attributes as asset sources.

Currently, Vite only processes assets in hardcoded HTML elements and attributes (<img src>, <video src>, etc.). Users cannot configure Vite to handle:

• Custom web components (e.g., <html-import src="...">)
• Non-standard attributes (e.g., data-src-dark, data-src-light)
  This causes asset URLs to not be transformed during build, resulting in broken paths.

Solution

Add a new html.additionalAssetSources configuration option that merges with the built-in asset sources list:

// vite.config.ts
export default defineConfig({
  html: {
    additionalAssetSources: {
      'html-import': { srcAttributes: ['src'] },
      img: { srcAttributes: ['data-src-dark', 'data-src-light'] },
    },
  },
})

For existing elements like img, additional attributes are merged with the defaults. In the example above, both the default src attribute and the custom data-src-* attributes will be processed.

Note

• Documentation: Added html.additionalAssetSources section in docs/config/shared-options.md

[monam2]

@bluwy I've updated the implementation to use the merge approach (instead of override) and also added documentation.
The CI is currently failing, but the failing tests (hmr-ssr.spec.ts on Windows) appear to be unrelated to my changes — they seem to be pre-existing flaky tests. Could you please take a look?

[bluwy]

Perfect 👍