Update cssparser and selectors dependencies to remove syn@1 transitive dependency

[emschwartz]

syn is a notoriously slow crate to build. Most of the Rust ecosystem has moved from syn v1 to v2. However, lol_html indirectly depends on syn@1 (lol_html depends on [email protected] and [email protected], which depend on [email protected] -> [email protected] -> syn@1). This likely means that most projects using lol_html are including both versions of syn.

It would be nice to remove the syn@1 dependency to speed up compilation time.

As an aside: thanks for open sourcing this library!