Modify synthetic default generation code for dual-mode module resolution (#46156)

tests/cases/conformance/node/nodeModulesCjsFormatFileAlwaysHasDefault.ts

@@ -0,0 +1,19 @@
+// @module: node12,nodenext
+// @declaration: true
+// @filename: subfolder/index.ts
+// cjs format file
+export const a = 1;
+// @filename: index.ts
+// esm format file
+import mod from "./subfolder/index.js";
+mod;
+// @filename: package.json
+{
+    "name": "package",
+    "private": true,
+    "type": "module"
+}
+// @filename: subfolder/package.json
+{
+    "type": "commonjs"
+}