[FIR generator] Factor out AbstractElement to common module

This is a step towards commonizing the code generator between
FIR and IR: KT-61970

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

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.generators.tree
+
+/**
+ * A common interface representing a FIR or IR tree element.
+ */
+interface AbstractElement<Element : AbstractElement<Element, Field>, Field : AbstractField> : FieldContainer, ImplementationKindOwner {
+
+    val name: String
+
+    val fields: Set<Field>
+
+    val parents: List<Element>
+
+    val typeArguments: List<TypeArgument>
+
+    val parentsArguments: Map<Element, Map<Importable, Importable>>
+
+    val overridenFields: Map<Field, Map<Importable, Boolean>>
+
+    val isSealed: Boolean
+        get() = false
+
+    override val allParents: List<ImplementationKindOwner>
+        get() = parents
+
+    override fun getTypeWithArguments(notNull: Boolean): String = type + generics
+}
+
+val AbstractElement<*, *>.generics: String
+    get() = typeArguments.takeIf { it.isNotEmpty() }
+        ?.let { it.joinToString(", ", "<", ">") { it.name } }
+        ?: ""