[FIR generator] Specify Field.isChild in base tree configuration

instead of inside implementation configuration.

Whether a field is a child element is a core concept of the tree,
not a matter of particular implementation class.

It is especially visible in IR tree, where the base classes implement the
acceptChildren method. It will also be more relevant in the future.

This also aligns the config style between IR and FIR tree generators.

^KT-65773 In Progress

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/AbstractField.kt

@@ -64,7 +64,7 @@ abstract class AbstractField<Field : AbstractField<Field>> {
      *
      * Only has effect if [containsElement] is `true`.
      */
-    var isChild: Boolean = true
+    abstract val isChild: Boolean
 
     open val overriddenTypes: MutableSet<TypeRefWithNullability> = mutableSetOf()
 
@@ -106,7 +106,6 @@ abstract class AbstractField<Field : AbstractField<Field>> {
         copy.visibility = visibility
         copy.fromParent = fromParent
         copy.useInBaseTransformerDetection = useInBaseTransformerDetection
-        copy.isChild = isChild
         copy.overriddenTypes += overriddenTypes
     }
 }

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/config/AbstractImplementationConfigurator.kt

@@ -368,19 +368,10 @@ abstract class AbstractImplementationConfigurator<Implementation, Element, Imple
                     withGetter = true
                 }
 
-            /**
-             * Whether this field semantically represents a reference to a child node of the tree.
-             *
-             * This has the effect of including or excluding this field from visiting it by visitors in the generated
-             * `acceptChildren` and `transformChildren` methods (child fields are always visited in those methods)
-             */
-            var isChild: Boolean = true
-
             fun applyConfiguration() {
                 field.withGetter = withGetter
                 field.customSetter = customSetter
                 isMutable?.let { field.isMutable = it }
-                field.isChild = isChild
 
                 when {
                     value != null -> field.defaultValueInImplementation = value