Add PostCSS plugin to fix relative @content and @plugin paths in @imported files (#14063)

We noticed an issue that happened when handling relative file imports in
the `@plugin` and the upcoming `@content` APIs. The problem arises from
relative files that are inside `@import`ed stylesheets. Take, for
example, the following folder structure:

```css
/* src/index.css */
@import "./dir/index.css";
```
```css
/* src/dir/index.css */
@plugin "../../plugin.ts";
```

It's expected that the path is relative to the CSS file that defined it.
However, right now, we use
[`postcss-import`](https://github.com/postcss/postcss-import) to flatten
the CSS file before running the tailwind build step. This causes these
custom-properties to be inlined in a flat file which removes the
information of which file is being referred:

```css
/* src/flat.css */
@plugin "../../plugin.ts"; /* <- This is now pointing to the wrong file */
```

There are generally two approaches that we can do to solve this:

1. **Handle `@import` flattening inside tailwindcss:** While generally
this would give us more freedom and less dependencies, this would
require some work to get all edge cases right. We need to support
layers/conditional imports and also handle all relative urls for
properties like `background-image`.
2. **Rewrite relative paths as a separate postcss visitor:** The
approach this PR takes is instead to implement a custom postcss plugin
that uses the AST to rewrite relative references inside `@plugin` and
`@content`. This has the benefit of requiring little changes to our
existing APIs. The rule is only enabled for relative references inside
`@plugin` and `@content`, so the surface of this rule is very small.

We can use this plugin inside all three current clients:

- `@tailwindcss/postcss` obviously already uses postcss
- `@tailwindcss/cli` also uses postcss to handle `@import` flattening
- `@tailwindcss/vite` allows us to add custom postcss rules via the CSS
pipeline. There are a few cases that we handle with care (e.g. in vite
you can pass a string to the postcss config which is supposed to load
the config from a file).

To validate the changes, we have added both a list of unit test cases to
the plugin itself as well as verified that all three clients are working
as expected:

- `@tailwindcss/postcss` now has an explicit test for this behavior
- `@tailwindcss/cli` and `@tailwindcss/vite` were manually tested by
updating the vite playground. The CLI was run with `--cwd
playgrounds/vite/ -i ./src/app.css -o foo.css`:
<img width="531" alt="Screenshot 2024-07-29 at 11 35 59"
src="https://github.com/user-attachments/assets/78f0acdc-a46c-4c6c-917a-2916417b1001">

packages/@tailwindcss-cli/src/commands/build/index.ts

@@ -1,5 +1,6 @@
 import watcher from '@parcel/watcher'
 import { IO, Parsing, scanDir, scanFiles, type ChangedContent } from '@tailwindcss/oxide'
+import fixRelativePathsPlugin from 'internal-postcss-fix-relative-paths'
 import { Features, transform } from 'lightningcss'
 import { existsSync } from 'node:fs'
 import fs from 'node:fs/promises'
@@ -259,6 +260,7 @@ function handleImports(
 
   return postcss()
     .use(atImport())
+    .use(fixRelativePathsPlugin())
     .process(input, { from: file })
     .then((result) => [
       result.css,