feat(model-api-gen): utility methods for adding typed root nodes

When adding typed nodes to the untyped root node, one always had to use
a considerable amount of boilerplate code to add the new node and get it
back typed:

```kotlin
root.addNewChild(null, C_Something.untyped()).typed<N_Something>()
```

This commit adds this logic as helper methods to model-api-gen-runtime
to simplify the process to

```kotlin
root.addNewChild(C_Something)
root.addNewChild("role", C_Something)
```

Author: languitar

model-api-gen-runtime/src/commonMain/kotlin/org/modelix/metamodel/ModelApiExt.kt

@@ -0,0 +1,23 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.modelix.metamodel
+
+import org.modelix.model.api.INode
+import org.modelix.model.api.addNewChild
+
+inline fun <reified NodeT : ITypedNode> INode.addNewChild(concept: INonAbstractConcept<NodeT>) =
+    addNewChild(null, concept.untyped()).typed<NodeT>()
+
+inline fun <reified NodeT : ITypedNode> INode.addNewChild(role: String, concept: INonAbstractConcept<NodeT>) =
+    addNewChild(role, concept.untyped()).typed<NodeT>()