Error character problems caused by pseudo-class content being escaped

Hello!

I found in my use that once the content of the pseudo-class has the '\\' symbol, it will be escaped to the wrong '' character, which is reproduced below.

```css
.iconfont:before {
content: "\e600";
}
```

Then run:

```javascript
const cssAST = csstree.parse(css)
const finalCSS = csstree.generate(cssPlain)
```

In the generated CSS, the expected result is escaped and will look like this:

```css
.iconfont:before{content:""}
```

Start with the supplied JavaScript reproduction using csstree.parse and csstree.generate, and compare the generated output with the CSS input containing content: "\e600". Done means generation preserves the escaped pseudo-class content instead of emitting the private-use character shown in the report.

Written by the indexing model from the issue text.