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_

The issue does not name a file, test, or entry point, so first locate the desktop PR changes view and its handling of .md files. Check how the existing raw file view and other rendered content are implemented. Done means a View control can open the full Markdown file in rendered form within the app on supported desktop platforms.

索引モデルが issue の本文から書いたものです。