[Feedback]: Markdown Preview with Mermaid Diagram support

What problem are you trying to solve?

I often create some markdown files in my project, sometimes these are docs, sometimes content collection, sometimes blog posts, sometimes design skill artifacts, etc ...

The file preview panel lets me view the .md file, but not in a rendered preview way.

What would improve it?

• render markdown previews richly
• support Mermaid diagrams
• allow for wider view (currently, seems restricted width so hard to review complete file).
• ^^ in fact my prompt chat panel area is the one I need to be the narrowest as that is a prompt and responsive. the File list and preview or browser are the areas I likely need to be wider

What do you do today?

If I check in to GitHub, I view there as it provides the preview rendering I need - or I use a VSCode extension to review.

Ideally, th file preview could support other text formats too: md, css, yaml, toml.

Product area

Other

该 issue 提到了文件预览面板，但没有指出文件、测试或实现入口。首先定位该面板，并梳理所请求的预览渲染、Mermaid 支持、更宽的布局以及其他文本格式；当范围达成一致，并且所选格式都有可用的预览时，工作才算完成。