feat: Support nested locale keys.

Example:

```js
{
  model: {
    user: {
      name: 'Real Name',
      createdAt: 'Joined At'
    }
  }
}
```

```js
ctx.__('model.user.name');
ctx.__('model.user.createdAt');
```

The before:

```js
{
  'model.user.name': 'Real Name',
  'model.user.createdAt', 'Joined At'
}
```

Benchmarks:

```
Deeps:  9
  2 tests completed.

  direct read a key x 85,993,593 ops/sec ±1.89% (96 runs sampled)
  by nested         x  4,203,837 ops/sec ±0.98% (93 runs sampled)
```

test/locales/zh-CN.js

@@ -1,4 +1,12 @@
 module.exports = {
   Email: '邮箱',
   'Hello %s, how are you today?': '%s，今天过得如何？',
+  model: {
+    user: {
+      fields: {
+        name: '姓名',
+        gender: '性别'
+      }
+    }
+  }
 };

test/locales/zh_TW.json

@@ -1,4 +1,12 @@
 {
   "Email": "郵箱",
-  "Hello %s, how are you today?": "%s，今天過得如何？"
+  "Hello %s, how are you today?": "%s，今天過得如何？",
+  "model": {
+    "user": {
+      "fields": {
+        "name": "姓名",
+        "gender": "性別"
+      }
+    }
+  }
 }