Function parse_rules 

fn parse_rules(
    content: &str,
    selector: String,
    iter: &mut Peekable<Chars<'_>>,
    paths: &mut FxIndexMap<String, CssPath>,
) -> Result<(), String>

This is used to parse inside a CSS {} block. If we encounter a new { inside it, we consider it as a new block and therefore recurse into parse_rules.