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

[goulvenclech]

format_css_selector_entry uses a placeholder for selectors :

/// 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: