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

plugins/js-plain-objects/compiler-plugin/testData/diagnostics/only-properties.kt

@@ -0,0 +1,15 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+
+// FILE: test.kt
+import kotlinx.js.JsPlainObject
+
+@JsPlainObject
+external interface Foo {
+    val foo: String
+    val bar: Int?
+    val fn: () -> String
+    val fnOptional: (() -> String)?
+    
+    <!METHODS_ARE_NOT_ALLOWED_INSIDE_JS_PLAIN_OBJECT!>fun test(): String<!>
+}