Fix output files warning by Turborepo (#74811)

The new version of Turborepo (rightfully) complains that two of our
packages don't have output files specified when running `pnpm build`:

```
 WARNING  no output files found for task @next/codemod#build. Please check your `outputs` key in `turbo.json`
 WARNING  no output files found for task @vercel/turbopack-next#build. Please check your `outputs` key in `turbo.json`
```

The package `@next/codemod` emits its `.js`, `.js.map`, and `.d.ts`
files into the same folder as the respective source files instead of
`dist` (as specified in the root `turbo.json`). And
`@vercel/turbopack-next` does not emit any output files. Both facts are
now reflected in the two added package-specific `turbo.json` files.

crates/next-core/js/turbo.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://turbo.build/schema.json",
+  "extends": ["//"],
+  "tasks": {
+    "build": {
+      "outputs": []
+    }
+  }
+}

packages/next-codemod/turbo.json

@@ -0,0 +1,26 @@
+{
+  "$schema": "https://turbo.build/schema.json",
+  "extends": ["//"],
+  "tasks": {
+    "build": {
+      "outputs": [
+        "**/*.js",
+        "**/*.d.ts",
+        "**/*.js.map",
+        "!bin/__testfixtures__/**",
+        "!transforms/__testfixtures__/**",
+        "!transforms/__tests__/**"
+      ]
+    },
+    "dev": {
+      "outputs": [
+        "**/*.js",
+        "**/*.d.ts",
+        "**/*.js.map",
+        "!bin/__testfixtures__/**",
+        "!transforms/__testfixtures__/**",
+        "!transforms/__tests__/**"
+      ]
+    }
+  }
+}