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

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

@@ -0,0 +1,21 @@
+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)
+
+    if (user.name != "Name") return "Fail: problem with `name` property"
+    if (user.age != 10) return "Fail: problem with `age` property"
+
+    val json = js("JSON.stringify(user)")
+    if (json != "{\"age\":10,\"name\":\"Name\"}") return "Fail: got the next json: $json"
+
+    return "OK"
+}