LibWeb: Parse :host() selector

We were already parsing non-function-syntax :host, so let's also do
the :host(...) variant. Note that we don't have matching for these yet.

This fixes many issues on sites generated by Wix, as they often have
selector lists that include some :host() selector, and we'd reject the
entire rule after failing to parse it.

Tests/LibWeb/Layout/input/css-host-selector-gets-parsed.html

@@ -0,0 +1,9 @@
+<style>
+:host, div { 
+    width: 100px;
+}
+:host(span), div { 
+    height: 100px;
+    background: green;
+}
+</style><div>whf