[K/JS] Prepare JS Plain Objects plugin to publication

plugins/js-plain-objects/compiler-plugin/testData/box/copy.kt

@@ -0,0 +1,25 @@
+package foo
+
+import kotlinx.js.JsPlainObject
+
+@JsPlainObject
+external interface User {
+    var name: String
+    val age: Int
+}
+
+fun box(): String {
+    val user = User(name = "Name", age = 10)
+    val copy = user.copy(age = 11)
+
+    if (copy === user) return "Fail: mutation instead of immutable copy"
+
+    val json = js("JSON.stringify(copy)")
+
+    if (copy.name != "Name") return "Fail: problem with copied `name` property"
+    if (copy.age != 11) return "Fail: problem with copied `age` property"
+
+    if (json != "{\"age\":11,\"name\":\"Name\"}") return "Fail: got the next json for the copy: $json"
+
+    return "OK"
+}