[Feature]: Support for GitHub markdown alert syntax

[chenjiahan]

What problem does this feature solve?

Alerts are a Markdown extension based on the blockquote syntax that you can use to emphasize critical information. On GitHub, they are displayed with distinctive colors and icons to indicate the significance of the content.

• Preview:

• Ref:

https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

What does the proposed API look like?

The same as GitHub