Escaped Class Names Using Vars Incorrectly Set to `undefined`

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Consider the following CSS:

```css
.bg-\\[url\\(https\\:\\/\\/example\\.com\\/image\\.png\\)\\] {
--un-url: url(https://example.com/image.png); background-image:var(--un-url);
}
```

The output from running the plugin is:

```css
.bg-\\[url\\(https\\:\\/\\/example\\.com\\/image\\.png\\)\\] {
background-image:undefined;
}
.bg-\\[url\\(https\\:\\/\\/example\\.com\\/image\\.png\\)\\]:\\/\\/example\\.com\\/image\\.png\\)\\]{
background-image:url(https://example.com/image.png);
}
```

The plugin seems to have an issue with escaping, or a variable directly following escaping.

Start by reproducing the reported CSS input through the PostCSS plugin and trace how escaped class names and custom-property values are parsed. Confirm the fix by ensuring the generated output keeps the complete escaped selector and emits the URL value instead of `undefined`.

Written by the indexing model from the issue text.