feat: Support union underlying type for TS/JS (#7961)

tests/ts/tsconfig.json

@@ -14,6 +14,7 @@
     "optional_scalars/**/*.ts",
     "namespace_test/**/*.ts",
     "union_vector/**/*.ts",
-    "arrays_test_complex/**/*.ts"
+    "arrays_test_complex/**/*.ts",
+    "union_underlying_type_test.ts"
   ]
 }