fix(build): map CSS chunks in chunk import maps (fix #22946)

[DebadityaHait]

Fixes #22946

When build.chunkImportMap is enabled, dynamic JavaScript dependencies use stable import-map specifiers, but extracted CSS dependencies still use content-hashed filenames. A CSS-only change can therefore leave a parent chunk filename unchanged while changing its cached preload array.

This tracks each emitted CSS asset's originating chunk, adds a corresponding stable CSS entry to the generated import map, and lets preload rewriting use that stable specifier. Builds without chunkImportMap are unchanged.

I considered rewriting only the preload array or changing global asset filenames. The former would not give the browser a target mapping, while the latter would affect unrelated builds. Extending the generated import map keeps the change scoped to the experimental option.

Tests:

• pnpm run build
• pnpm run test-build chunk-importmap
• pnpm exec eslint packages/vite/src/node/plugins/css.ts packages/vite/src/node/plugins/html.ts playground/chunk-importmap/__tests__/chunk-importmap.spec.ts

[sapphi-red]

Thanks!

I added a new test case and some comments clarifying the behavior.