[Feature request]

**Is your feature request related to a problem? Please describe.**
I noticed for many files the code runner button can tell what the file type is open and perform the right action based on that. But markdown does nothing. Its understandable that markdown files are not code but its frustrating when I see a play button but it doesn't open the markdown preview for me if I'm viewing a markdown content in edit mode.

**Describe the solution you'd like**
Having the code runner button open markdown preview :)

首先定位针对文件类型的 code-runner 处理逻辑，以及用于预览 Markdown 的 VS Code 命令。验证在编辑模式下打开 Markdown 文件时的行为，然后确认激活 code-runner 按钮会打开 Markdown 预览，同时不会改变其他文件类型的行为。

由索引模型根据 Issue 内容生成。