feat: add a new post content styles (#132)

添加新的内容渲染样式。https://github.com/sindresorhus/github-markdown-css

<img width="1031" alt="图片" src="https://github.com/halo-dev/theme-earth/assets/21301288/a9a59389-3fd9-4675-861c-54d5021391ac">

Fixes https://github.com/halo-dev/theme-earth/issues/42

/kind feature

```release-note
添加新的内容渲染样式
```

settings.yaml

@@ -89,6 +89,15 @@ spec:
     - group: post
       label: 文章
       formSchema:
+        - $formkit: select
+          name: content_style
+          label: 内容样式
+          value: "tailwind"
+          options:
+            - label: GitHub 风格
+              value: github
+            - label: Tailwind CSS Typography
+              value: tailwind
         - $formkit: repeater
           name: share_items
           label: 分享按钮