add '...@append' method to block properties

doc/en/block-properties.md

@@ -213,3 +213,11 @@ User properties must be declared in `pack:config/user-props.toml` file:
 ```
 
 Example: [user properties of pack **base**](../../res/content/base/config/user-props.toml).
+
+## Methods
+
+Methods are used to manage the overwriting of properties when extending a block with other packs.
+
+### `property_name@append`
+
+Adds elements to the end of the list instead of completely overwriting it.

doc/ru/block-properties.md

@@ -224,3 +224,11 @@
 ```
 
 Пример: [пользовательские свойства пака **base**](../../res/content/base/config/user-props.toml).
+
+## Методы
+
+Методы используются для управлением перезаписью свойств при расширении блока другими паками.
+
+### `имя_свойства@append`
+
+Добавляет элементы в конец списка, вместо его полной перезаписи.

src/content/ContentBuilder.cpp

@@ -93,8 +93,8 @@ std::unique_ptr<Content> ContentBuilder::build() {
         def->rt.surfaceReplacement = content->blocks.require(def->surfaceReplacement).rt.id;
         if (def->properties == nullptr) {
             def->properties = dv::object();
+            def->properties["name"] = def->name;
         }
-        def->properties["name"] = def->name;
     }
 
     for (ItemDef* def : itemDefsIndices) {

src/content/ContentLoader.cpp

@@ -187,11 +187,49 @@ static void perform_user_block_fields(
     layout = StructLayout::create(fields);
 }
 
+static void process_method(
+    dv::value& properties,
+    const std::string& method,
+    const std::string& name,
+    const dv::value& value
+) {
+    if (method == "append") {
+        if (!properties.has(name)) {
+            properties[name] = dv::list();
+        }
+        auto& list = properties[name];
+        if (value.isList()) {
+            for (const auto& item : value) {
+                list.add(item);
+            }
+        } else {
+            list.add(value);
+        }
+    } else {
+        throw std::runtime_error(
+            "unknown method " + method + " for " + name
+        );
+    }
+}
+
 void ContentLoader::loadBlock(
     Block& def, const std::string& name, const fs::path& file
 ) {
     auto root = files::read_json(file);
-    def.properties = root;
+    if (def.properties == nullptr) {
+        def.properties = dv::object();
+        def.properties["name"] = name;
+    }
+    for (auto& [key, value] : root.asObject()) {
+        auto pos = key.rfind('@');
+        if (pos == std::string::npos) {
+            def.properties[key] = value;
+            continue;
+        }
+        auto field = key.substr(0, pos);
+        auto suffix = key.substr(pos + 1);
+        process_method(def.properties, suffix, field, value);
+    }
 
     if (root.has("parent")) {
         const auto& parentName = root["parent"].asString();