…` in optimized builds (#20576)

## Summary

Fixes #20570

Vite adds its own `postcss-import` before the PostCSS plugins of the
user. Thus, when `@tailwindcss/postcss` runs, the `@import`-ed
stylesheets are already inlined. Their relative `url(…)`s are still
relative to their own file. At the end, Vite resolves each `url(…)`
against the `source.input.file` of its declaration.

Without optimization, we keep the `source` of each node (since
#17775), so this works.
With optimization, we parse the output of Lightning CSS again, and every
node gets the input file as its `source`. `optimize` is on by default
when `NODE_ENV` is `production`, and `vite build` sets it. Vite then
looks for the asset next to the input file, does not find it, and keeps
the URL as it is:

```
./fonts/ExampleSans.woff2 referenced in ./fonts/ExampleSans.woff2 didn't resolve at build time, it will remain unchanged to be resolved at runtime
```

This is the problem that
#16636 and
#16700 reported.
#16965 fixed URL
rewriting only for the stylesheets that we inline ourselves.
`transformAssetUrls`
(#18321) has no effect
here.

## Solution

When we optimize, we now rewrite the relative `url(…)`s of declarations
that come from another file, so that they are relative to the input
file. This is the same rewrite that we do for the stylesheets that we
inline ourselves. After the rewrite, the URLs match the `source` that
the optimized nodes get.

- We rewrite the AST that we get from PostCSS, before we compile it. The
stylesheets that we load ourselves are not in that AST yet, so we do not
rewrite their URLs two times.
- Without optimization, nothing changes, because the nodes keep their
`source`.
- `transformAssetUrls: false` disables the rewrite.

To share the code, `rewriteUrls` in `@tailwindcss/node` now uses a new
`rewriteUrlsInAst` function. A callback gives it the base folder of each
declaration.

This also changes optimized builds of other setups that run
`postcss-import` before `@tailwindcss/postcss`, like `postcss-cli`.
Their URLs are now relative to the input file, like the URLs of the
stylesheets that we inline ourselves.

## Test plan

- Added unit tests in `packages/@tailwindcss-postcss/src/index.test.ts`.
They run `postcss-import` before the plugin with `optimize` on, with
`optimize` off, and with `transformAssetUrls: false`. The test with
`optimize` on fails on `main`.
- Added an integration test in
`integrations/postcss/url-rewriting.test.ts`. It runs `vite build` with
`@tailwindcss/postcss` in a PostCSS config. Without this change, Vite
emits no fonts and the test fails. With this change, it passes.
- `pnpm vitest run`: 5598 passed.
- `pnpm vitest --root=./integrations run` on `postcss/url-rewriting`,
`vite/url-rewriting`, `postcss/index`, `postcss/source`,
`postcss/plugins`, `postcss/config`, `postcss/core-as-postcss-plugin`
and `postcss/multi-root`: all passed. I did not run
`postcss/next.test.ts`.
- Built a Vite 8.2.0 project with a font CSS file in a package and one
in a local folder. With `optimize: true`, the fonts are now emitted to
`dist/assets` and the URLs are rewritten, the same as with `optimize:
false`.