`crates/csslsrs/src/features/hover.rs`: HTML selectors preview

`format_css_selector_entry` uses a placeholder for selectors :
```rs
/// Formats the CSS selector entry into a hover content string.
fn format_css_selector_entry(name: &str, specificity: Option<(u32, u32, u32)>) -> String {
let mut content = String::new();
// TODO: this is a placeholder, we should render an HTML preview of the selector
writeln!(content, "**{}**\n", escape_markdown(name)).unwrap();
// Add specificity if available
if let Some((ids, classes, elements)) = specificity {
writeln!(content,
"[Selector Specificity](https://developer.mozilla.org/docs/Web/CSS/Specificity): ({}, {}, {})\n",
ids, classes, elements
).unwrap();
}
content
}
```

An HTML preview should replace it: