LibWeb: Make CSS 'An+B' parsing spec-compliant

Parsing this pattern from CSS tokens turns out to be slightly crazy, but
thankfully well documented in the spec.

The spec lists the cases in order of simple -> complicated, but this
would cause problems in code, since `<n-dimension> <signed-.integer>`
would never by reached, as `<n-dimension>` comes before. Instead, I
have grouped them by their first token.

Also renamed the NthChildPattern class to ANPlusBPattern, to match spec
terminology.

Userland/Libraries/LibWeb/CSS/Parser/Token.cpp

@@ -54,7 +54,7 @@ String Token::to_debug_string() const
     case Type::Number:
         builder.append("Number: ");
         builder.append(m_value.to_string());
-        builder.append(m_unit.to_string());
+        builder.append(m_number_type == NumberType::Integer ? " (int)" : " (float)");
         return builder.to_string();
     case Type::Percentage:
         builder.append("Percentage: ");