fix: make sealed classes abstract inside d.ts files.

js/js.translator/testData/typescript-export/declarations/declarations.d.ts

@@ -62,7 +62,7 @@ declare namespace JS_TESTS {
                 get x(): number;
             };
         }
-        class TestSealed {
+        abstract class TestSealed {
             protected constructor(name: string);
             get name(): string;
         }

js/js.translator/testData/typescript-export/declarations/declarations.kt

@@ -216,7 +216,7 @@ class OuterClass {
 }
 
 @JsExport
-class KT38262 {
+open class KT38262 {
     fun then(): Int = 42
     fun catch(): Int = 24
 }