JS support for PostCSS config

### Command

config

### Description

Currently the CLI supports custom PostCSS configurations in the format of JSON files. The problem is that the JSON format has its limitation. In our case for example we try to configure CSS purging with the `fullhuman/postcss-purgecss` plugin.

```json
{
"plugins": {
"@fullhuman/postcss-purgecss": {
"content": ["**/*.html", "**/*.ts", "**/*.js"],
"skippedContentGlobs": ["node_modules/**"],
}
}
}
```

Unfortunately this config is not enough and therefore we would need to configure some extractors:

```javascript
import purgeJs from "purgecss-from-js";
import purgeHtml from "purgecss-from-html";

const options = {
content: [], // files to extract the selectors from
css: [], // css
extractors: [
{
extractor: purgeJs,
extensions: ["js"],
},
{
extractor: purgeHtml,
extensions: ["html"],
},
],
};
export default options;
```

### Describe the solution you'd like
Support PostCSS configuration files writen in JavaScript.

### Describe alternatives you've considered
_No response_

Start at the CLI's config command and trace how JSON PostCSS configuration files are located, loaded, and passed to PostCSS. Review the existing configuration tests before deciding where JavaScript module loading belongs. Done means a JavaScript PostCSS config can be discovered and used with custom extractor functions, while existing JSON configuration still works.

索引モデルが issue の本文から書いたものです。