fix(node): stub missing process.sourceMapsEnabled (#31358)

See https://nodejs.org/api/process.html#processsourcemapsenabled

This fixes an issue in Fresh with vite's environment API which relies on
this property to check if vite should map stack traces
https://github.com/vitejs/vite/blob/main/packages/vite/src/module-runner/sourcemap/index.ts#L17

tests/unit_node/process_test.ts

@@ -1207,6 +1207,14 @@ Deno.test({
   },
 });
 
+Deno.test({
+  name: "process.sourceMapsEnabled",
+  fn() {
+    // @ts-ignore: not available in the types yet.
+    assertEquals(process.sourceMapsEnabled, true);
+  },
+});
+
 // Regression test for https://github.com/denoland/deno/issues/23761
 Deno.test({
   name: "process.uptime without this",