chore: ignore __tests__ in prod builds (#8307)

The tests are being bundled and shipped in prod, this adds a bit of unneeded weight to npm installs. Now they won't be included.

```
@react-navigation/core
- before: 274 files - pkg: 211.0 kB - unpkg: 1 MB
- after: 238 files - pkg: 192.1 kB - unpkg: 827.3 kB
```

packages/stack/package.json

@@ -24,7 +24,8 @@
   "types": "lib/typescript/src/index.d.ts",
   "files": [
     "src",
-    "lib"
+    "lib",
+    "!**/__tests__"
   ],
   "sideEffects": false,
   "publishConfig": {