feat(manifest): add assets field for standalone CSS entry points

[adamkoppede]

The manifest.json file lists imported assets and CSS files per entry point, so the back end code can generate corresponding <link rel=preload> and <link rel=stylesheet>. These two manifest properties were missing for pure CSS entry points, if they weren't included by some other JavaScript entry point.

See: #18392 (comment)

[sapphi-red]

Thank you

[adamkoppede]

With #21136 merged in 46d3077 ("docs: clarify manifest.json imports field is JS chunks only (#21136)"), this change also needs to update docs/guide/backend-integration.md and packages/vite/src/node/plugins/manifest.ts.

[adamkoppede]

I tried a quick update of this branch to v8.0.0-beta.1 but couldn't get it working yet. I found the injectChunkMetadata workaround for viteMetadata, but could not figure out why generateBundle() of the vite:manifest JavaScript plugin is no longer called (in pnpm test-build backend-integration). Perhaps the implementation from rolldown is used instead?

I only pushed the conflict resolution for the docs for now and converted the PR to Draft since it doesn't work on version 8.

[sapphi-red]

The JS plugin is not called unless experimental.enableNativePlugin is disabled. It's ok to push the change as long as the tests passes with the _VITE_TEST_JS_PLUGIN=1 env var.

[adamkoppede]

Do you want to wait with the non-_VITE_TEST_JS_PLUGIN=1 half of this change until vite:css-post uses builtin:vite-css-post?

[shulaoda]

@sapphi-red The parts of the native manifest related to viteMetadata are all on the JS side, so I made the changes directly in this PR.

[shulaoda]

Do you want to wait with the non-_VITE_TEST_JS_PLUGIN=1 half of this change until vite:css-post uses builtin:vite-css-post?

I’ve polished up the native manifest part, thanks for your contribution.