feat(css): export PostCSS config type for type-safe configs

[linyiru]

Description

Re-exports the PostCSS user config shape as PostcssUserConfig from vite, so PostCSS configs can be typed against the same definition Vite loads them with:

import type { PostcssUserConfig } from 'vite'

const config: PostcssUserConfig = { plugins: [] }
export default config

Closes #19109 (approved by @bluwy in the issue). This continues the work of #22525, which was closed by its author; the only outstanding review feedback there (from @sapphi-red) was "We should use the types from postcss-load-config" instead of hand-writing the shape — this PR addresses exactly that.

Why it mirrors the type instead of re-exporting it

A literal re-export turned out not to be possible with the current type-bundling setup, for two compounding reasons:

• postcss-load-config is a bundled devDependency, not a runtime dependency — so it can't be referenced as an external import('postcss-load-config') in the published .d.ts (consumers don't have it installed).
• Its declarations use CommonJS export = syntax, which rolldown-plugin-dts cannot re-export through the bundled .d.ts. A named re-export fails codegen (Export 'PostcssUserConfig' is not defined), and resolving the type inline pulls in non-externalized postcss/lib/* subpaths and fails on their export = declarations.

So PostcssUserConfig is defined by mirroring postcss-load-config's Config using the already-public postcss types. To make sure the mirror can never silently drift, a type-level test asserts it stays structurally identical to the upstream Config:

export type cases = [ExpectTrue<Equal<PostcssUserConfig, PostcssLoadConfig>>]

This also caught a subtle bug in the previous attempt: it used PostCSS.TransformCallback where Config actually uses Transformer (which additionally requires postcssPlugin/postcssVersion).

Tests

• packages/vite/src/node/__tests_dts__/postcss.ts — a type-only test that (1) locks PostcssUserConfig to postcss-load-config's Config via Equal, and (2) checks both the array and object plugin formats are accepted and an invalid value is rejected. It runs as part of pnpm typecheck (tsc -p src/node/__tests_dts__).
• Verified locally: pnpm typecheck, pnpm build-types (roll + check), eslint, and oxfmt --check all pass.

Notes for reviewers

• The mirror + Equal lock is the approach I landed on given the export = / bundled-dep constraints above. If you'd prefer a literal reference (e.g. by also externalizing postcss subpaths in rolldown.dts.config.ts, which leaks import ... from "postcss/lib/processor" into the public .d.ts), happy to switch — let me know.
• No docs change included: the type carries inline JSDoc with a usage example. Glad to add a docs note if you'd like one.