Provide a way to select only code

[NavyAdmiral]

Much like how GitHub is already doing it. The -, + and line number views in the PR Description page should be isolated from the code selection.

[NavyAdmiral]

@rebornix why isn't this view using the default vscode diff view?

[rebornix]

@NavyAdmiral the description page is a pure web app in a webview and it's contributed by the extension, which means it can't use vscode's own diff view.

We can probably use Monaco editor for this case.