LibWeb: Add PropertyID::Custom to code generator

Author: https://github.com/TobyAsE
Commit: 8396099e0e
Pull-request: https://github.com/SerenityOS/serenity/pull/7455
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/CodeGenerators/Generate_CSS_PropertyID_h.cpp

@@ -53,6 +53,7 @@ namespace Web::CSS {
 
 enum class PropertyID {
     Invalid,
+    Custom,
 )~~~");
 
     json.value().as_object().for_each_member([&](auto& name, auto& value) {