perf: add autofocus to login username input (#742)

#### What type of PR is this?
/kind improvement

#### What this PR does / why we need it:
以便用户可以直接输入用户名，不需要再进行额外的鼠标键盘操作
#### Which issue(s) this PR fixes:
Fixes https://github.com/halo-dev/halo/issues/2779

#### Screenshots:
![image](https://user-images.githubusercontent.com/28836989/204964637-07cb3b3f-efbe-4045-991e-a73698176dd1.png)


```release-note
Console 端登录页面支持自动聚焦输入框
```

src/components/login/LoginForm.vue

@@ -86,6 +86,7 @@ onMounted(() => {
       }"
       name="username"
       placeholder="用户名"
+      :autofocus="true"
       type="text"
       validation="required"
     >