Add Markdown View and rendering support

### Feature summary

The ability to view .md files rendered, not just raw markdown format.

### What problem are you trying to solve?

When I look at the changes section of a PR, I can see that the .md files are shown in the raw markdown format, but without the ability to view them rendered. So, I'm requesting a feature to give the ability to view these .md (markdown) files in the same Github Copilot app on windows (as well as other supported desktop platforms).

To make it even more clear. In github.com, when viewing changes, There's a dropdown menu button almost on the top-right, where you can View the .md file (Where this shows the Whole file including the changes made).
This feature here is what I would like to request you to add in the Github Copilot app on windows, as well as other dektop platforms.

Thanks for this app.

### Proposed solution

New "View" button on any markdown (.md) file being viewed in the changes section of a PR.
This new button should View the Full .md file including the changes, and View it Rendered, not just raw markdown format.

### Workflow impact

Any one has a repository (whether it's private or public), would be able to start or continue sessions from github copilot app, as well as viewing files such as Markdown files rendered without the need to go out to some other app or to the github.com website to just view that markdown file.

### Installation context

_No response_

### Additional context

_No response_

该 issue 没有指定文件、测试或入口点，因此首先定位 desktop PR changes view 及其对 .md 文件的处理方式。检查现有 raw file view 和其他已渲染内容是如何实现的。完成标准是：View 控件可以在受支持的 desktop 平台上于应用内以渲染形式打开完整的 Markdown 文件。

由索引模型根据 Issue 内容生成。