fix(model-api): make SimpleProperty a data class

Among other advantages, this makes error messages with simple properties easier to read because of the overridden toString().

Author: mhuster23

model-api/src/commonMain/kotlin/org/modelix/model/api/SimpleProperty.kt

@@ -15,7 +15,7 @@ package org.modelix.model.api
 
 import kotlin.jvm.JvmOverloads
 
-class SimpleProperty
+data class SimpleProperty
 @JvmOverloads constructor(
     private val simpleName: String,
     override val isOptional: Boolean = true,