[FE] Introduce a language feature entry for expect actual classes

^KT-62885 Fixed

compiler/testData/diagnostics/tests/multiplatform/expectActualClassesAreInBeta_suppressWarningWithLv.kt

@@ -0,0 +1,51 @@
+// FIR_IDENTICAL
+// ENABLE_EXPECT_ACTUAL_CLASSES_WARNING
+// LANGUAGE: +ExpectActualClasses
+// MODULE: m1-common
+// FILE: common.kt
+
+expect class Clazz {
+    class Nested
+
+    fun memberFun()
+    val memberProp: Clazz
+}
+
+expect interface Interface
+
+expect object Object
+
+expect annotation class Annotation
+
+expect enum class Enum
+
+expect class ActualTypealias
+
+expect fun function()
+
+expect val property: Clazz
+
+// MODULE: m2-jvm()()(m1-common)
+// FILE: jvm.kt
+actual class Clazz {
+    actual class Nested
+
+    actual fun memberFun() {}
+    actual val memberProp: Clazz = null!!
+}
+
+actual interface Interface
+
+actual object Object
+
+actual annotation class Annotation
+
+actual enum class Enum
+
+actual typealias ActualTypealias = ActualTypealiasImpl
+
+class ActualTypealiasImpl
+
+actual fun function() {}
+
+actual val property: Clazz = null!!