[FIR generator] Move Field and FieldContainer 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/AbstractField.kt

@@ -0,0 +1,49 @@
+/*
+ * 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
+
+abstract class AbstractField : Importable {
+
+    abstract val name: String
+
+    open val arguments = mutableListOf<Importable>()
+
+    abstract val nullable: Boolean
+
+    abstract var isVolatile: Boolean
+
+    abstract var isFinal: Boolean
+
+    abstract var isLateinit: Boolean
+
+    abstract var isParameter: Boolean
+
+    open val arbitraryImportables: MutableList<Importable> = mutableListOf()
+
+    open var optInAnnotation: ArbitraryImportable? = null
+
+    abstract var isMutable: Boolean
+    open val withGetter: Boolean get() = false
+    open val customSetter: String? get() = null
+
+    var fromParent: Boolean = false
+
+    override fun toString(): String {
+        return name
+    }
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other == null) return false
+        if (javaClass != other.javaClass) return false
+        other as AbstractField
+        return name == other.name
+    }
+
+    override fun hashCode(): Int {
+        return name.hashCode()
+    }
+}

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

@@ -0,0 +1,11 @@
+/*
+ * 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
+
+interface FieldContainer {
+    val allFields: List<AbstractField>
+    operator fun get(fieldName: String): AbstractField?
+}