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

core/compiler.common/src/org/jetbrains/kotlin/descriptors/ClassKind.kt

@@ -32,3 +32,6 @@ inline val ClassKind.isInterface: Boolean
 
 inline val ClassKind.isEnumClass: Boolean
     get() = this == ClassKind.ENUM_CLASS
+
+inline val ClassKind.isClass: Boolean
+    get() = this == ClassKind.CLASS

core/descriptors/src/org/jetbrains/kotlin/resolve/DescriptorUtils.kt

@@ -136,6 +136,11 @@ val DeclarationDescriptor.isInsidePrivateClass: Boolean
         return parent != null && DescriptorVisibilities.isPrivate(parent.visibility)
     }
 
+val DeclarationDescriptor.isInsideInterface: Boolean
+    get() {
+        val parent = containingDeclaration as? ClassDescriptor
+        return parent != null && parent.kind.isInterface
+    }
 
 fun ClassDescriptor.getSuperClassNotAny(): ClassDescriptor? {
     for (supertype in defaultType.constructor.supertypes) {