feat(@JsExport for interfaces): add ability to export interfaces in TypeScript.

compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.kt

@@ -18,11 +18,23 @@ val String.extensionProperty<!>
 annotation class <!WRONG_EXPORTED_DECLARATION("annotation class")!>AnnotationClass<!>
 
 @JsExport
-interface <!WRONG_EXPORTED_DECLARATION("interface")!>SomeInterface<!>
+interface SomeInterface
 
 @JsExport
 external interface GoodInterface
 
+@JsExport
+interface InterfaceWithCompanion {
+    companion <!WRONG_EXPORTED_DECLARATION("companion object inside exported interface")!>object<!> {
+        fun foo() = 42
+    }
+}
+
+@JsExport
+interface OuterInterface {
+    class <!WRONG_EXPORTED_DECLARATION("nested class inside exported interface")!>Nested<!>
+}
+
 @JsExport
 value class <!WRONG_EXPORTED_DECLARATION("value class")!>A(val a: Int)<!>