`postcss` is imported but not declared as a dependency

`lib/fix-nesting-at-rule.mjs` imports `postcss`, but the published `package.json` (6.0.2) only declares:

```json
"dependencies": {
"postcss-selector-parser": "^7.1.1"
},
"peerDependencies": {
"stylelint": ">= 16.9.0"
}
```

With pnpm's default isolated `node_modules`, loading the plugin fails with `Cannot find package 'postcss'`.

I assume this has gone unnoticed because `postcss` is already in your lockfile through stylelint (dependabot keeps bumping it there, e.g. #33/#37), and npm users get it through hoisting since stylelint depends on postcss anyway. It only breaks under isolated layouts.

Fix would be declaring `postcss` as a dependency (or a peer next to `stylelint`). In the meantime we're patching it on our side:

```yaml
# pnpm-workspace.yaml
packageExtensions:
stylelint-use-nesting@*:
dependencies:
postcss: ^8.5.15
```

Happy to send a PR if you'd take one.

Start with lib/fix-nesting-at-rule.mjs and the published package.json, checking how the plugin imports postcss and how dependencies are declared. Install the package with pnpm's isolated node_modules and load the plugin; done means it resolves postcss without a package extension or transitive dependency.

Written by the indexing model from the issue text.